U.S. patent application number 11/485420 was filed with the patent office on 2007-04-05 for semiconductor light-emitting device.
This patent application is currently assigned to HITACHI CABLE, LTD.. Invention is credited to Katsuya Akimoto, Masahiro Arai, Kazuyuki Iizuka, Taichiroo Konno.
Application Number | 20070075327 11/485420 |
Document ID | / |
Family ID | 37901052 |
Filed Date | 2007-04-05 |
United States Patent
Application |
20070075327 |
Kind Code |
A1 |
Arai; Masahiro ; et
al. |
April 5, 2007 |
Semiconductor light-emitting device
Abstract
A semiconductor light emitting device has a light-emitting
portion formed on a semiconductor substrate, an As-based p-type
contact layer formed thereon, a current spreading layer formed
thereon of a metal oxide material, and a buffer layer formed
between the p-type cladding layer and the p-type contact layer. The
buffer layer has a group III/V semiconductor with a p-type
conductivity and hydrogen or carbon included intentionally or
unavoidably therein, and the buffer layer has a thickness equal to
or greater than a diffusion length L of a dopant doped into the
p-type contact layer.
Inventors: |
Arai; Masahiro; (Tsuchiura,
JP) ; Konno; Taichiroo; (Tsuchiura, JP) ;
Iizuka; Kazuyuki; (Tsuchiura, JP) ; Akimoto;
Katsuya; (Tsuchiura, JP) |
Correspondence
Address: |
FOLEY AND LARDNER LLP;SUITE 500
3000 K STREET NW
WASHINGTON
DC
20007
US
|
Assignee: |
HITACHI CABLE, LTD.
|
Family ID: |
37901052 |
Appl. No.: |
11/485420 |
Filed: |
July 13, 2006 |
Current U.S.
Class: |
257/103 ;
257/E33.005 |
Current CPC
Class: |
H01L 33/30 20130101;
H01L 33/14 20130101 |
Class at
Publication: |
257/103 |
International
Class: |
H01L 33/00 20060101
H01L033/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 30, 2005 |
JP |
2005-285921 |
Sep 30, 2005 |
JP |
2005-285931 |
Claims
1. A semiconductor light emitting device, comprising: a
light-emitting portion formed on a semiconductor substrate, the
light-emitting portion comprising an n-type cladding layer, an
active layer and a p-type cladding layer; an As-based p-type
contact layer formed on the light-emitting portion, the p-type
contact layer being doped with a dopant at a concentration of
1.times.10.sup.19/cm.sup.3 or more and the dopant material doped
into the p-type contact layer is different from one doped into the
p-type cladding layer; a current spreading layer formed on the
p-type contact layer, the current spreading layer comprising a
metal oxide material; and a buffer layer formed between the p-type
cladding layer and the p-type contact layer, wherein the buffer
layer comprises a group III/V semiconductor with a p-type
conductivity and hydrogen included intentionally or unavoidably
therein, and the buffer layer comprises a thickness equal to or
greater than a diffusion length L of a dopant doped into the p-type
contact layer.
2. The semiconductor light emitting device according to claim 1,
wherein: the p-type cladding layer contains Mg as the dopant, the
p-type contact layer contains Zn as the dopant, and the diffusion
length L is represented by: L
[.mu.m]=6.869.times.10.sup.-15.times.N.sub.H.sup.0.788, where
N.sub.H is a hydrogen concentration [cm.sup.-3] of the buffer
layer.
3. The semiconductor light emitting device according to claim 1,
wherein: the p-type contact layer comprises Al.sub.xGa.sub.1-xAs,
where 0.ltoreq.x.ltoreq.0.4.
4. The semiconductor light emitting device according to claim 1,
wherein: the buffer layer comprises Al.sub.xGa.sub.1-xAs, where
0.4.ltoreq.x.ltoreq.1.
5. The semiconductor light emitting device according to claim 1,
wherein: the current-spreading layer comprises at least one of ITO
(indium tin oxide), SnO.sub.2 (tin oxide), ATO (antimony tin
oxide), In.sub.2O.sub.3 (indium oxide), ZnO (zinc oxide), GZO
(gallium zinc oxide), BZO (boron zinc oxide), AZO (aluminum zinc
oxide), CdO (cadmium oxide), CTO (cadmium tin oxide), IZO (indium
zinc oxide).
6. The semiconductor light emitting device according to claim 1,
wherein: the current-spreading layer comprises a thickness of
within .+-.30% of d calculated by:
d=A.times..lamda..sub.p/(4.times.n), where A is a constant (A=1 or
3), .lamda..sub.p is an emission wavelength (nm) of the light
emitting device, and n is a refractive index of the
current-spreading layer.
7. The semiconductor light emitting device according to claim 1,
wherein: the light-emitting portion comprises
(Al.sub.xGa.sub.1-x).sub.yIn.sub.1-yP, where 0.ltoreq.x.ltoreq.1
and 0.4.ltoreq.y.ltoreq.0.6, and the p-type cladding layer and the
n-type cladding layer comprise a higher Al composition than the
active layer.
8. The semiconductor light emitting device according to claim 1,
wherein: the current-spreading layer comprises a carrier
concentration of 7.times.10.sup.20/cm.sup.3 or more.
9. The semiconductor light emitting device according to claim 1,
wherein: the p-type contact layer comprises a thickness of 1 nm or
more and 30 nm or less.
10. The semiconductor light emitting device according to claim 1,
further comprising: a light reflecting layer formed between the
substrate and the n-type cladding layer, wherein the light
reflecting layer comprises 10 pairs or more and 30 pairs or less of
semiconductor layers, each pair comprising a combination of a
high-refractive index material layer and a low-refractive index
material layer.
11. The semiconductor light emitting device according to claim 10,
wherein: the light reflecting layer comprises at least one of
(Al.sub.xGa.sub.1-x)As where 0.4.ltoreq.x.ltoreq.1, and
(Al.sub.xGa.sub.1-x).sub.yIn.sub.1-yP where 0.ltoreq.x.ltoreq.1 and
0.4.ltoreq.y.ltoreq.0.6.
12. The semiconductor light emitting device according to claim 1,
wherein: the active layer comprises a light emitting layer and a
barrier layer with a bandgap wider than the light emitting
layer.
13. The semiconductor light emitting device according to claim 12,
wherein: the active layer comprises a quantum well structure that
the light emitting layer comprises a thickness of 9 nm or less, or
a strained quantum well structure that the light emitting layer
comprises a crystal lattice constant different from that of the
n-type cladding layer or the p-type cladding layer.
14. The semiconductor light emitting device according to claim 1,
wherein: the p-type cladding layer comprises at least a portion
with a Mg concentration of 1.times.10.sup.17/cm.sup.3 or more and
5.times.10.sup.18/cm.sup.3 or less.
15. The semiconductor light emitting device according to claim 1,
wherein: the substrate comprises a semiconductor material of GaAs,
Ge or Si, or a metallic material with a thermal conductivity
greater than Si.
16. The semiconductor light emitting device according to claim 1,
further comprising: a diffusion-suppressing layer formed between
the active layer and the p-type cladding layer, wherein the
diffusion-suppressing layer comprises any one or a combination of:
an undoped semiconductor layer, a semiconductor layer with a lower
dopant concentration than the p-type cladding layer, and a
semiconductor layer doped with an n-type dopant and a p-type dopant
together to be neutral in pseudo conduction type, and the
diffusion-suppressing layer comprises a thickness of 300 nm or
less.
17. The semiconductor light emitting device according to claim 1,
further comprising: a diffusion-suppressing layer formed between
the active layer and the n-type cladding layer, wherein the
diffusion-suppressing layer comprises any one or a combination of:
an undoped semiconductor layer, a semiconductor layer with a lower
dopant concentration than the n-type cladding layer, and a
semiconductor layer doped with an n-type dopant and a p-type dopant
together to be neutral in pseudo conduction type, and the
diffusion-suppressing layer comprises a thickness of 200 nm or
less.
18. A semiconductor light emitting device, comprising: a
light-emitting portion formed on a semiconductor substrate, the
light-emitting portion comprising an n-type cladding layer, an
active layer and a p-type cladding layer; an As-based p-type
contact layer formed on the light-emitting portion, the p-type
contact layer being doped with a dopant at a concentration of
1.times.10.sup.19/cm.sup.3 or more and the dopant doped into the
contact layer being different from that doped into the p-type
cladding layer; a current spreading layer formed on the p-type
contact layer, the current spreading layer comprising a metal oxide
material; and a buffer layer formed between the p-type cladding
layer and the p-type contact layer, wherein the buffer layer
comprises a group III/V semiconductor with a p-type conductivity
and carbon included intentionally or unavoidably therein, and the
buffer layer comprises a thickness equal to or greater than a
diffusion length L of a dopant doped into the p-type contact
layer.
19. The semiconductor light emitting device according to claim 18,
wherein: the p-type cladding layer comprises Mg as the dopant, the
p-type contact layer comprises Zn as the dopant, and the diffusion
length L is represented by: L
[.mu.m]=6.872.times.10.sup.-14.times.N.sub.C.sup.0.733, where
N.sub.C is a carbon concentration [cm.sup.-3] of the buffer
layer.
20. The semiconductor light emitting device according to claim 18,
wherein: the p-type contact layer comprises Al.sub.xGa.sub.1-xAs,
where 0.ltoreq.x.ltoreq.0.4.
21. The semiconductor light emitting device according to claim 18,
wherein: the buffer layer comprises Al.sub.xGa.sub.1-xAs, where
0.4.ltoreq.x.ltoreq.1.
22. The semiconductor light emitting device according to claim 18,
wherein: the current-spreading layer comprises at least one of ITO
(indium tin oxide), SnO.sub.2 (tin oxide), ATO (antimony tin
oxide), In.sub.2O.sub.3 (indium oxide), ZnO (zinc oxide), GZO
(gallium zinc oxide), BZO (boron zinc oxide), AZO (aluminum zinc
oxide), CdO (cadmium oxide), CTO (cadmium tin oxide), IZO (indium
zinc oxide).
23. The semiconductor light emitting device according to claim 18,
wherein: the current-spreading layer comprises a thickness of
within .+-.30% of d calculated by:
d=A.times..lamda..sub.p/(4.times.n), where A is a constant (A=1 or
3), .lamda..sub.p is an emission wavelength (nm) of the light
emitting device, and n is a refractive index of the
current-spreading layer.
24. The semiconductor light emitting device according to claim 18,
wherein: the light-emitting portion comprises
(Al.sub.xGa.sub.1-x).sub.yIn.sub.1-yP, where 0.ltoreq.x.ltoreq.1
and 0.4.ltoreq.y.ltoreq.0.6, and the p-type cladding layer and the
n-type cladding layer comprise a higher Al composition than the
active layer.
25. The semiconductor light emitting device according to claim 18,
wherein: the current-spreading layer comprises a carrier
concentration of 7.times.10.sup.20/cm.sup.3 or more.
26. The semiconductor light emitting device according to claim 18,
wherein: the p-type contact layer comprises a thickness of 1 nm or
more and 30 nm or less.
27. The semiconductor light emitting device according to claim 18,
further comprising: a light reflecting layer formed between the
substrate and the n-type cladding layer, wherein the light
reflecting layer comprises 10 pairs or more and 30 pairs or less of
semiconductor layers, each pair comprising a combination of a
high-refractive index material layer and a low-refractive index
material layer.
28. The semiconductor light emitting device according to claim 27,
wherein: the light reflecting layer comprises at least one of
(Al.sub.xGa.sub.1-x)As where 0.4.ltoreq.x.ltoreq.1, and
(Al.sub.xGa.sub.1-x).sub.yIn.sub.1-yP where 0.ltoreq.x.ltoreq.1 and
0.4.ltoreq.y.ltoreq.0.6.
29. The semiconductor light emitting device according to claim 18,
wherein: the active layer comprises a light emitting layer and a
barrier layer with a bandgap wider than the light emitting
layer.
30. The semiconductor light emitting device according to claim 29,
wherein: the active layer comprises a quantum well structure that
the light emitting layer comprises a thickness of 9 nm or less, or
a strained quantum well structure that the light emitting layer
comprises a crystal lattice constant different from that of the
n-type cladding layer or the p-type cladding layer.
31. The semiconductor light emitting device according to claim 18,
wherein: the p-type cladding layer comprises at least a portion
with a Mg concentration of 1.times.10.sup.17/cm.sup.3 or more and
5.times.10.sup.18/cm.sup.3 or less.
32. The semiconductor light emitting device according to claim 18,
wherein: the substrate comprises a semiconductor material of GaAs,
Ge or Si, or a metallic material with a thermal conductivity
greater than Si.
33. The semiconductor light emitting device according to claim 18,
further comprising: a diffusion-suppressing layer formed between
the active layer and the p-type cladding layer, wherein the
diffusion-suppressing layer comprises any one or a combination of:
an undoped semiconductor layer, a semiconductor layer with a lower
dopant concentration than the p-type cladding layer, and a
semiconductor layer doped with an n-type dopant and a p-type dopant
together to be neutral in pseudo conduction type, and the
diffusion-suppressing layer comprises a thickness of 300 nm or
less.
34. The semiconductor light emitting device according to claim 18,
further comprising: a diffusion-suppressing layer formed between
the active layer and the n-type cladding layer, wherein the
diffusion-suppressing layer comprises any one or a combination of:
an undoped semiconductor layer, a semiconductor layer with a lower
dopant concentration than the n-type cladding layer, and a
semiconductor layer doped with an n-type dopant and a p-type dopant
together to be neutral in pseudo conduction type, and the
diffusion-suppressing layer comprises a thickness of 200 nm or
less.
Description
[0001] The present application is based on Japanese patent
application Nos.2005-285921 and 2005-285931, the entire contents of
which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention relates to a semiconductor light emitting
device and, in particular, to a high-brightness semiconductor light
emitting device with a transparent conductive film to serve as a
current spreading layer.
[0004] 2. Description of the Related Art
[0005] In recent years, the crystalline quality of GaN-based or
AlGaInP-based semiconductors is improved since they can be grown by
a MOVPE (metalorganic vapor phase epitaxy) method. Thus, a
high-brightness blue, green, orange, yellow, and red light-emitting
diode (herein referred to as LED) as a semiconductor light emitting
device can be fabricated.
[0006] However, in order to achieve the high brightness, the
current spreading property needs to be improved such that current
is uniformly supplied into a chip plane of an LED. For example, an
AlGaInP-based LED device is fabricated such that the current
spreading layer has a thickness increased to about 5 to 10 .mu.m.
Therefore, the growth of the thick current spreading layer causes
an increase in the fabrication cost of the LED device. Thus, the
LED device is difficult to fabricate at low cost.
[0007] In consideration of this, a method is proposed that an ITO
(indium tin oxide) or ZnO (zinc oxide) film is used as the current
spreading layer to get a sufficient translucency and good current
spreading characteristics (JP-A-8-83927). Further, a method is
proposed that an ITO film is directly formed on a p-type cladding
layer (U.S. Reissued Pat. No.35665).
[0008] When the ITO film is used as the current spreading layer,
the conventional method of increasing the thickness of the
semiconductor layer as the current spreading layer to about 5 to 10
.mu.m is not necessary, and the formation of the epitaxial layer
can be saved by that much. Thus, the high-brightness LED device and
the epitaxial wafer for the LED device can be fabricated at low
cost.
[0009] However, when the ITO film is used as a window layer, a
contact resistance is generated between the semiconductor layer and
the ITO film of a metal oxide, and a forward voltage
disadvantageously increases. More specifically, the ITO film used
as a transparent conductive film (transparent electrode) is an
n-type semiconductor. On the other hand, the upper cladding layer
contacting the ITO film is a p-type semiconductor. Therefore, when
a forward voltage is applied to the LED, a reverse bias is
established between the transparent conductive film (transparent
electrode) and the p-type cladding layer. Because of this, a large
voltage (i.e., increased operating voltage) has to be applied to
flow current therethrough.
[0010] To solve this problem, a method is proposed that a high
carrier concentration layer (=contact layer) is formed to contact
the ITO film to offer a tunnel junction which allows the LED to be
driven at a low voltage (e.g., Reissue Pat. No. 35665).
[0011] However, the contact layer needs to be formed as a high
carrier concentration layer and a thin film since it has to offer
the tunnel junction and can be a light-absorbing layer to light
emitted from the active layer. Therefore, the dopant diffusion is
likely to occur due to heat generated during the growth. As a
result, the following two problems will be caused.
[0012] First, the dopant is diffused from the contact layer to the
depth direction of the LED device. When the dopant reaches the
active layer of the LED device, the dopant causes a defect in the
active layer. The defect will work as a nonradiative recombination
component to lower the optical output of the LED device.
[0013] Second, since a substantial carrier concentration of the
high-carrier concentration contact layer lowers due to the dopant
diffusion, the tunnel junction is difficult to obtain and the
tunnel voltage is increased. For this reason, the drive voltage
(forward voltage) of the LED device disadvantageously
increases.
[0014] Zn is widely used as a p-type dopant for AlGaInP-based or
AlGaAs-based compound semiconductors, but it is known that its
diffusion constant is relatively large and it causes an adverse
effect during heat process. Therefore, when Zn is doped into the
p-type cladding layer to increase a carrier concentration thereof,
the Zn is diffused into the active layer so that the
characteristics of the LED device are deteriorated.
[0015] Further, when Mg, which has a smaller diffusion constant
than Zn, is used as a p-type dopant of the p-type cladding layer to
increase a carrier concentration thereof, and Zn, which can
relatively easy offer a carrier concentration of
1.times.10.sup.19/cm.sup.3 and a sufficiently small contact
resistivity, is used as a p-type dopant of the p-type contact
layer, the diffusion of the p-type dopants, Zn and Mg causes a
significant adverse effect since they are likely to be
inter-diffused.
[0016] Meanwhile, there are three ways of doping the dopants, i.e.,
Mg only, Zn only and a combination of Zn and Mg. The amount of
diffusion is increased in this order. Namely, the magnitude
relation in diffusion amount is (Mg only)<(Zn only)<(a
combination of Zn and Mg). Thus, in order to suppress the
interdiffusion, only Zn or Mg is desirably doped. However, to dope
only Zn or Mg causes the next advantages and disadvantages.
[0017] In case of doping Mg only, it is difficult to provide the
contact layer with a high carrier concentration. For example, it is
very difficult to offer the tunnel junction relative to the ITO
film. On the other hand, the Mg diffusion from the p-type cladding
layer to the active layer is very low so that an LED device with a
long lifetime can be stably obtained.
[0018] In case of doping Zn only, in contrast to the case of doping
Mg only, the tunnel junction relative to the ITO film can be
relatively easy obtained. Namely, it is relatively easy to increase
the carrier concentration of the contact layer. On the other hand,
since the Zn diffusion from the p-cladding layer to the active
layer is likely to occur, the lifetime of LED device becomes short
as compared to the case of doping Mg only. Further, in case of
doping Zn, it is difficult to obtain a high carrier concentration
crystal for AlGaInP-based material as compared to Mg. Thus, a range
of Zn carrier concentration to be set is limited and therefore it
is difficult to obtain a high-brightness LED device.
[0019] In case of doping a combination of Zn and Mg, an LED device
can be obtained suitably to some extent by using the following
composition. First, Zn is used as a dopant for the contact layer to
get the tunnel junction relative to the ITO film. Then, Mg is used
as a dopant for a p-type semiconductor layer other than the contact
layer, e.g., buffer layer and p-type cladding layer, to have the
high-carrier concentration p-type semiconductor layer to offer a
high-brightness LED device.
[0020] However, the combination of Zn and Mg causes the
interdiffusion of Zn and Mg as described earlier. Therefore, in
this case, it is necessary to suppress the degradation of the
device lifetime.
[0021] On the other hand, when the contact layer is formed directly
on the p-type cladding layer without forming the buffer layer and
the ITO film is formed thereon (U.S. Reissued Pat. No.35665), the
dopant is likely to reach the active layer due to the thin p-type
cladding layer, whereby the device lifetime will be shortened.
Further, due to the thin p-type cladding layer, the device is
likely to be broken by damage in wire bonding.
SUMMARY OF THE INVENTION
[0022] It is an object of the invention to provide a semiconductor
light emitting device that can achieve a high brightness and low
operating voltage while preventing a reduction in optical output
and an increase in operating voltage with time. [0023] (1)
According to one aspect of the invention, a semiconductor light
emitting device comprises:
[0024] a light-emitting portion formed on a semiconductor
substrate, the light-emitting portion comprising an n-type cladding
layer, an active layer and a p-type cladding layer;
[0025] an As-based p-type contact layer formed on the
light-emitting portion, a carrier concentration of the p-type
contact layer comprising 1.times.10.sup.19/cm.sup.3 or more and a
dopant material of the p-type contract layer being different from
that of the p-type cladding layer;
[0026] a current spreading layer formed on the p-type contact
layer, the current spreading layer comprising a metal oxide
material; and
[0027] a buffer layer formed between the p-type cladding layer and
the p-type contact layer,
[0028] wherein the buffer layer comprises a group III/V
semiconductor with a p-type conductivity and hydrogen included
intentionally or unavoidably therein, and
[0029] the buffer layer comprises a thickness equal to or greater
than a diffusion length L of a dopant doped into the p-type contact
layer.
[0030] Herein, to include "intentionally" means to dope positively
or purposely. To include "unavoidably" means an unavoidable
phenomenon that an impurity such as H (hydrogen) and C (carbon) is
naturally mixed into a crystal although it is not doped positively,
purposely or intentionally. Further, herein "undoped", "non-doped"
or "not doped" means that a dopant is not doped positively,
intentionally or purposely, and it does not exclude a phenomenon
that an impurity such as H (hydrogen) and C (carbon) is unavoidably
mixed in a crystal.
[0031] In the above invention (1), the following modifications and
changes can be made.
[0032] (i) The p-type cladding layer contains Mg as the dopant, the
p-type contact layer contains Zn as the dopant, and the diffusion
length L is represented by: L
[.mu.m]=6.869.times.10.sup.-15.times.N.sub.H.sup.0.788, where
N.sub.H is a hydrogen concentration [cm.sup.-3] of the buffer
layer.
[0033] (ii) The p-type contact layer comprises
Al.sub.xGa.sub.1-xAs, where 0.ltoreq.x.ltoreq.0.4.
[0034] Semiconductor materials capable of providing stably the
contact layer with a high carrier concentration of
1.0.times.10.sup.19/cm.sup.3 or more are limited. Of them, an
optimal semiconductor material is a Zn-doped Al.sub.xGa.sub.1-xAs,
where 0.ltoreq.x.ltoreq.0.4. However, since the AlGaAs is not
transparent to the emission wavelength, it needs to be formed with
a thickness of about 30 nm or less.
[0035] (iii) The buffer layer comprises Al.sub.xGa.sub.1-xAs, where
0.4.ltoreq.x.ltoreq.1.
[0036] (iv) The current-spreading layer comprises at least one of
ITO (indium tin oxide), SnO.sub.2 (tin oxide), ATO (antimony tin
oxide), In.sub.2O.sub.3 (indium oxide), ZnO (zinc oxide), GZO
(gallium zinc oxide), BZO (boron zinc oxide), AZO (aluminum zinc
oxide), CdO (cadmium oxide), CTO (cadmium tin oxide), IZO (indium
zinc oxide).
[0037] (v) The current-spreading layer comprises a thickness of
within .+-.30% of d calculated by:
d=A.times..lamda..sub.p/(4.times.n), where A is a constant (A=1 or
3), .lamda..sub.p is an emission wavelength (nm) of the light
emitting device, and n is a refractive index of the
current-spreading layer.
[0038] (vi) The light-emitting portion comprises
(Al.sub.xGa.sub.1-x).sub.yIn.sub.1-yP, where 0.ltoreq.x.ltoreq.1
and 0.4.ltoreq.y.ltoreq.0.6, and
[0039] the p-type cladding layer and the n-type cladding layer
comprise a higher Al composition than the active layer.
[0040] (vii) The current-spreading layer comprises a carrier
concentration of 7.times.10.sup.20/cm.sup.3 or more.
[0041] (viii) The p-type contact layer comprises a thickness of 1
nm or more and 30 nm or less.
[0042] (ix) The semiconductor light emitting device further
comprises:
[0043] a light reflecting layer formed between the substrate and
the n-type cladding layer,
[0044] wherein the light reflecting layer comprises 10 pairs or
more and 30 pairs or less of semiconductor layers, each pair
comprising a combination of a high-refractive index material layer
and a low-refractive index material layer.
[0045] (x) The light reflecting layer comprises at least one of
(Al.sub.xGa.sub.1-x)As where 0.4.ltoreq.x.ltoreq.1, and
(Al.sub.xGa.sub.1-x).sub.yIn.sub.1-yP where 0.ltoreq.x.ltoreq.1 and
0.4.ltoreq.y.ltoreq.0.6.
[0046] (xi) The active layer comprises a light emitting layer and a
barrier layer with a bandgap wider than the light emitting
layer.
[0047] (xii) The active layer comprises a quantum well structure
that the light emitting layer comprises a thickness of 9 nm or
less, or a strained quantum well structure that the light emitting
layer comprises a crystal lattice constant different from that of
the n-type cladding layer or the p-type cladding layer.
[0048] (xiii) The p-type cladding layer comprises at least a
portion with a Mg concentration of 1.times.10.sup.17/cm.sup.3 or
more and 5.times.10.sup.18/cm.sup.3 or less.
[0049] If the Mg concentration is less than
1.times.10.sup.17/cm.sup.3, the carrier concentration of the p-type
cladding layer is too low to get its sufficient effect as a carrier
supply layer. If the Mg is doped more than
5.times.10.sup.18/cm.sup.3, crystal defects will arise in the
p-type cladding layer nearly according to the Mg concentration so
that the diffusion of the dopant causes a reduction of the internal
quantum efficiency of the LED, thus the optical output in the LED
device lowers.
[0050] (xiv) The substrate comprises a semiconductor material of
GaAs, Ge or Si, or a metallic material with a thermal conductivity
greater than Si.
[0051] (xv) The semiconductor light emitting device further
comprises:
[0052] a diffusion-suppressing layer formed between the active
layer and the p-type cladding layer,
[0053] wherein the diffusion-suppressing layer comprises any one or
a combination of: an undoped semiconductor layer, a semiconductor
layer with a lower dopant concentration than the p-type cladding
layer, and a semiconductor layer doped with an n-type dopant and a
p-type dopant together to be neutral in pseudo conduction type,
and
[0054] the diffusion-suppressing layer comprises a thickness of 300
nm or less.
[0055] (xvi) The semiconductor light emitting device further
comprises:
[0056] a diffusion-suppressing layer formed between the active
layer and the n-type cladding layer,
[0057] wherein the diffusion-suppressing layer comprises any one or
a combination of: an undoped semiconductor layer, a semiconductor
layer with a lower dopant concentration than the n-type cladding
layer, and a semiconductor layer doped with an n-type dopant and a
p-type dopant together to be neutral in pseudo conduction type,
and
[0058] the diffusion-suppressing layer comprises a thickness of 200
nm or less. [0059] (2) According to another aspect of the
invention, a semiconductor light emitting device comprises:
[0060] a light-emitting portion formed on a semiconductor
substrate, the light-emitting portion comprising an n-type cladding
layer, an active layer and a p-type cladding layer;
[0061] an As-based p-type contact layer formed on the
light-emitting portion, a concentration of the p-type contact layer
comprising 1.times.10.sup.19/cm.sup.3 or more and the dopant
material of p-type contact layer being different from that of the
p-type cladding layer;
[0062] a current spreading layer formed on the p-type contact
layer, the current spreading layer comprising a metal oxide
material; and
[0063] a buffer layer formed between the p-type cladding layer and
the p-type contact layer,
[0064] wherein the buffer layer comprises a group III/V
semiconductor with a p-type conductivity and carbon included
intentionally or unavoidably therein, and
[0065] the buffer layer comprises a thickness equal to or greater
than a diffusion length L of a dopant doped into the p-type contact
layer.
[0066] In the above invention (2), the following modifications and
changes can be made.
[0067] (xvii) The p-type cladding layer contains Mg as the dopant,
the p-type contact layer contains Zn as the dopant, and the
diffusion length L is represented by: L
[.mu.m]=6.872.times.10.sup.-14.times.N.sub.C.sup.0.733, where
N.sub.C is a carbon concentration [cm.sup.-3] of the buffer
layer.
[0068] (xviii) The p-type contact layer comprises
Al.sub.xGa.sub.1-xAs, where 0.ltoreq.x.ltoreq.0.4.
[0069] Semiconductor materials capable of providing stably the
contact layer with a high carrier concentration of
1.0.times.10.sup.19/cm.sup.3 or more are limited. Of them, an
optimal semiconductor material is a Zn-doped Al.sub.xGa.sub.1-xAs,
where 0.ltoreq.x.ltoreq.0.4. However, since the AlGaAs is not
transparent to the emission wavelength, it needs to be formed with
a thickness of about 30 nm or less.
[0070] (xix) The buffer layer comprises Al.sub.xGa.sub.1-xAs, where
0.4.ltoreq.x.ltoreq.1.
[0071] (xx) The current-spreading layer comprises at least one of
ITO (indium tin oxide), SnO.sub.2 (tin oxide), ATO (antimony tin
oxide), In.sub.2O.sub.3 (indium oxide), ZnO (zinc oxide), GZO
(gallium zinc oxide), BZO (boron zinc oxide), AZO (aluminum zinc
oxide), CdO (cadmium oxide), CTO (cadmium tin oxide), IZO (indium
zinc oxide).
[0072] (xxi) The current-spreading layer comprises a thickness of
within .+-.30% of d calculated by:
d'=A.times..lamda..sub.p/(4.times.n), where A is a constant (A=1 or
3), .lamda..sub.p is an emission wavelength (nm) of the light
emitting device, and n is a refractive index of the
current-spreading layer.
[0073] (xxii) The light-emitting portion comprises
(Al.sub.xGa.sub.1-x).sub.yIn.sub.1-yP, where 0.ltoreq.x.ltoreq.1
and 0.4.ltoreq.y.ltoreq.0.6, and
[0074] the p-type cladding layer and the n-type cladding layer
comprise a higher Al composition than the active layer.
[0075] (xxiii) The current-spreading layer comprises a carrier
concentration of 7.times.10.sup.20/cm.sup.3 or more.
[0076] (xxiv) The p-type contact layer comprises a thickness of 1
nm or more and 30 nm or less.
[0077] (xxv) The semiconductor light emitting device further
comprises:
[0078] a light reflecting layer formed between the substrate and
the n-type cladding layer,
[0079] wherein the light reflecting layer comprises 10 pairs or
more and 30 pairs or less of semiconductor layers, each pair
comprising a combination of a high-refractive index material layer
and a low-refractive index material layer.
[0080] (xxvi) The light reflecting layer comprises combinations of
at least one of (Al.sub.xGa.sub.1-x)As where 0.4.ltoreq.x.ltoreq.1,
and (Al.sub.xGa.sub.1-x).sub.yIn.sub.1-yP where 0.ltoreq.x.ltoreq.1
and 0.4.ltoreq.y.ltoreq.0.6.
[0081] (xxvii) The active layer comprises a light emitting layer
and a barrier layer with a bandgap wider than the light emitting
layer.
[0082] (xxviii) The active layer comprises a quantum well structure
that the light emitting layer comprises a thickness of 9 nm or
less, or a strained quantum well structure that the light emitting
layer comprises a crystal lattice constant different from that of
the n-type cladding layer or the p-type cladding layer.
[0083] (xxix) The p-type cladding layer comprises at least a
portion with a Mg concentration of 1.times.10.sup.17/cm.sup.3 or
more and 5.times.10.sup.18/cm.sup.3 or less.
[0084] If the Mg concentration is less than
1.times.10.sup.17/cm.sup.3, the carrier concentration of the p-type
cladding layer is too low to get its sufficient effect as a carrier
supply layer. If the Mg is doped more than
5.times.10.sup.18/cm.sup.3, crystal defects will arise in the
p-type cladding layer nearly according to the Mg concentration so
that the diffusion of the dopant causes a reduction of the internal
quantum efficiency of the LED, thus the optical output of the LED
device lowers.
[0085] (xxx) The substrate comprises a semiconductor material of
GaAs, Ge or Si, or a metallic material with a thermal conductivity
greater than Si.
[0086] (xxxi) The semiconductor light emitting device further
comprises:
[0087] a diffusion-suppressing layer formed between the active
layer and the p-type cladding layer,
[0088] wherein the diffusion-suppressing layer comprises any one or
a combination of: an undoped semiconductor layer, a semiconductor
layer with a lower dopant concentration than the p-type cladding
layer, and a semiconductor layer doped with an n-type dopant and a
p-type dopant together to be neutral in pseudo conduction type,
and
[0089] the diffusion-suppressing layer comprises a thickness of 300
nm or less.
[0090] (xxxii) The semiconductor light emitting device further
comprises:
[0091] a diffusion-suppressing layer formed between the active
layer and the n-type cladding layer,
[0092] wherein the diffusion-suppressing layer comprises any one or
a combination of: an undoped semiconductor layer, a semiconductor
layer with a lower dopant concentration than the n-type cladding
layer, and a semiconductor layer doped with an n-type dopant and a
p-type dopant together to be neutral in pseudo conduction type,
and
[0093] the diffusion-suppressing layer comprises a thickness of 200
nm or less.
ADVANTAGES OF THE INVENTION
[0094] In the invention, the diffusion length of Zn can be set by a
concentration value of H which is included intentionally or
unavoidably in the buffer layer. Thus, the Zn diffusion length can
be set so that it falls within the thickness of the buffer
layer.
[0095] In the invention, by suppressing effectively the
interdiffusion of dopants from the p-type contact layer and the
other p-type semiconductor layer, a semiconductor light emitting
device can be provided to achieve a high brightness a low operating
voltage and a good reliability by suppressing a reduction in
optical output and an increase in operating voltage with time.
BRIEF DESCRIPTION OF THE DRAWINGS
[0096] The preferred embodiments according to the invention will be
explained below referring to the drawings, wherein:
[0097] FIG. 1 is a schematic cross sectional view showing an
AlGaInP-based red LED in first and second preferred embodiments of
the invention and Examples 1 and 5 of the invention;
[0098] FIG. 2A is a graph showing a relationship between an H
concentration of a buffer layer and a diffusion length of Zn in
Examples 1-4 of the invention;
[0099] FIG. 2B is a graph showing a relationship between a C
concentration of a buffer layer and a diffusion length of Zn in
Examples 5-8 of the invention;
[0100] FIG. 3 is a schematic cross sectional view showing an
AlGaInP-based red LED in Examples 2 and 6 of the invention;
[0101] FIG. 4 is a schematic cross sectional view showing an
AlGaInP-based red LED in Examples 3 and 7 of the invention;
[0102] FIG. 5 is a schematic cross sectional view showing an
AlGaInP-based red LED in Examples 4 and 8 of the invention;
[0103] FIG. 6 is a graph showing a relationship between a thickness
of a contact layer and a transmittance at an LED emission
wavelength;
[0104] FIG. 7 is a diagram showing reflectance spectra of an ITO
film formed on a GaAs substrate;
[0105] FIG. 8 is a graph showing a relationship between the number
of pairs in light reflecting layer and a perpendicular
reflectance;
[0106] FIG. 9 is a schematic cross sectional view showing an
AlGaInP-based red LED in Comparative Examples 1 and 3;
[0107] FIG. 10 is a schematic cross sectional view showing an
AlGaInP-based red LED in Comparative Examples 2 and 4;
[0108] FIG. 11 is a diagram showing a result of SIMS analysis in
Comparative Examples 1 and 3;
[0109] FIG. 12 is a diagram showing a result of SIMS analysis in
Examples 1 and 5 of the invention;
[0110] FIG. 13A is a graph showing a relationship between a V/III
ratio during the growth of a buffer layer and an H concentration in
the buffer layer; and
[0111] FIG. 13B is a graph showing a relationship between a V/III
ratio during the growth of a buffer layer and a C concentration in
the buffer layer.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
First Embodiment
[0112] FIG. 1 is a cross sectional view showing an AlGaInP-based
red LED in the first preferred embodiment of the invention.
[0113] The LED comprises, sequentially formed on an n-type GaAs
substrate 1 as a semiconductor substrate, an n-type GaAs buffer
layer 2, an n-type AlGaInP cladding layer 3 (herein also simply
called n-type cladding layer), an undoped AlGaInP active layer 4
(herein also simply called active layer) and a p-type AlGaInP
cladding layer 5 (herein also simply called p-type cladding layer
5), where the layers 3-5 are epitaxially grown to compose a
light-emitting portion. Further, a p-type (As-based) AlGaAs contact
layer 7 (herein also simply called p-type contact layer 7) doped
with a p-type dopant at a high concentration is formed on the
p-type cladding layer 5. Further, an ITO film (current-spreading
layer) 8, which is a current-spreading layer made of a metal oxide
and a transparent conductive film, is formed on the p-type contact
layer 7. A surface electrode 9 is formed on the surface of the ITO
film 8, and a back surface electrode 10 is formed on the back
surface of the n-type GaAs substrate 1.
[0114] The p-type contact layer 7 is made of Al.sub.xGa.sub.1-xAS
(wherein 0.ltoreq.x.ltoreq.0.4), and has a thickness of 1 nm or
more and 30 nm or less. Zn as a p-type dopant is doped at a high
carrier concentration of 1.times.10.sup.19/cm.sup.3 or more.
[0115] The film thickness of the ITO film 8 as the current
spreading layer is in a range of .+-.30% of d calculated by a
relational expression of d=A.times..lamda..sub.p/(4.times.n), where
A is a constant (A=1 or 3), .lamda..sub.p is an emission wavelength
(nm) of the LED, and n is a refractive index of the ITO film. The
ITO film 8 as the current spreading layer is formed by a vacuum
deposition method or a sputtering method. The ITO film 8 has
preferably a carrier concentration of 7.times.10.sup.20/cm.sup.3 or
more just after the film formation.
[0116] A feature of the LED is that a p-type buffer layer 6 is
formed between the p-type contact layer 7 and the p-type cladding
layer 5, and it comprises a group III/V semiconductor doped with
the Mg as a p-dopant.
[0117] The p-type buffer layer 6 is formed of an
Al.sub.xGa.sub.1-xAs (0.4.ltoreq.x.ltoreq.1) which is optically
transparent to the emission wavelength and lattice-matched to the
AlGaInP-based material to compose the light-emitting portion. The
p-type buffer layer 6 has a thickness t of a diffusion length or
more of Zn doped in the p-type contact layer 7.
[0118] The p-type buffer layer 6 is an AlGaAs layer with a high Al
composition, optically transparent to the emission wavelength of an
LED device composed of an AlGaInP-based material, easier to grow
its crystal than a four-element material such as AlGaInP, and
almost lattice-matched to the AlGaInP-based material to compose the
light-emitting portion. Thus, the p-type buffer layer 6 is formed
of the material capable of lowering the operating voltage of the
LED device.
[0119] The diffusion length of Zn doped in the p-type contact layer
7 has, as shown in FIG. 2A, a correlation with a concentration of H
(hydrogen) contained in the p-type buffer layer 6. The Zn diffusion
length is represented by an expression:
L=6.869.times.10.sup.-15.times.N.sub.H.sup.0.788, where N.sub.H is
an H concentration [cm.sup.-3] in the buffer layer and L is a
diffusion length [.mu.m] of a dopant doped in the p-type contact
layer 7.
[0120] In this embodiment, the H concentration in the p-type buffer
layer 6 is set such that the thickness t of the p-type buffer layer
6 is greater than the Zn diffusion length L obtained by the above
expression or from the curve in FIG. 2A. The setting of H
concentration is closely controlled in consideration of parameters
such as the V/III ratio during the growth of the p-type buffer
layer 6 or Al composition of the p-type buffer layer 6.
[0121] For example, when the p-type buffer layer 6 has a thickness
of 5 .mu.m, the H concentration in the p-type buffer layer 6 is set
to be 3.times.10.sup.18/cm.sup.3. The diffusion length of Zn doped
in the p-type contact layer 7 is obtained by the above expression
or from the curve in FIG. 2A to be about 2.5 .mu.m, which falls
within the thickness of the p-type buffer layer 6. Thereby, the Zn
diffusion from the p-type contact layer 7 can be prevented, and,
therefore, the interdiffusion between Zn of the p-type contact
layer 7 and Mg of the other p-type semiconductor layers 5, 6 can be
effectively prevented.
[0122] The inventors found that the interdiffusion reaction between
Zn of the p-type contact layer 7 and Mg of the p-type cladding
layer 5 and the p-type buffer layer 6 is in close relation with the
H concentration in the p-type buffer layer 6, and that this
phenomenon is significant especially when the p-type buffer layer 6
comprises an AlGaAs layer with a high Al composition. Therefore,
since the H concentration of the p-type buffer layer 6 can be set
by controlling closely the parameters such as the V/III ratio
during the growth of the p-type buffer layer 6 or Al composition of
the p-type buffer layer 6, the thickness of the p-type buffer layer
6 can be suitably set so as to control the interdiffusion distance
between Zn and Mg, and a long lifetime LED device can be
obtained.
[0123] In this embodiment, between the n-type GaAs substrate 1 and
the n-type cladding layer 3, e.g., between the n-type GaAs buffer
layer 3 and the n-type cladding layer 3, a light reflecting layer
may be formed which comprises 10 pairs or more and 30 pairs or less
of two semiconductor layers different in refractive index, made of
high-refractive index material and low-refractive index material,
respectively.
[0124] Further, in this embodiment, between the active layer 4 and
the p-type cladding layer 5 or between the active layer 4 and the
n-type cladding layer 3, a diffusion-suppressing layer may be
formed ,which comprises any one or a combination of an undoped
semiconductor layer, a semiconductor layer with a lower dopant
concentration than the p-type cladding layer 5, and a semiconductor
layer doped with an n-type dopant and a p-type dopant
simultaneously to be neutral in pseudo conduction type. The
diffusion-suppressing layer has desirably a thickness of 300 nm or
less when it is inserted between the active layer 4 and the p-type
cladding layer 5. Alternatively, it has desirably a thickness of
200 nm or less when it is inserted between the active layer 4 and
the n-type cladding layer 3.
[0125] The reasons for employing the LED structure as described in
this embodiment will be explained below.
[0126] (1) First, the thickness of the AlGaAs-based buffer layer 6
of the embodiment is set based on that it needs to prevent the
dopant doped into the p-type semiconductor from penetrating into
the active layer 4 due to the interdiffusion between Zn doped in
the p-type contact layer 7 and Mg doped in the other p-type
semiconductor layers 5, 6 so as to offer an LED device with a long
lifetime and a high reliability.
[0127] The inventors found that the interdiffusion between Zn and
Mg is in close relation with the concentration of H contained in
the p-type buffer layer 6, and that the diffusion length (or
diffusion distance) of the impurities can be controlled by the
concentration of H contained in the p-type buffer layer 6. This can
be known by the experimental result to indicate the relationship
between the H concentration in the p-type buffer layer 6 and the
diffusion length of Zn as shown in FIG. 2A. Based on this result,
the diffusion length of Zn doped in the p-type contact layer 7 can
be obtained by the expression:
L=6.869.times.10.sup.-15.times.N.sub.H.sup.0.788, where N.sub.H is
an H concentration [cm.sup.-3] of the buffer layer and L is a
diffusion length [.mu.m] of a dopant doped in the p-type contact
layer 7. Thus, in the embodiment, since the AlGaAs buffer layer 6
has a thickness t equal to or more than the Zn diffusion length L
obtained by the expression, the LED device can achieve a good
property both in initial characteristics and reliabilities.
[0128] Meanwhile, it is not always necessary to specify the upper
limit of the thickness t of the buffer layer 6. In other words, as
the thickness is increased from the suitable thickness (i.e., the
thickness t), the following situations will occur.
[0129] The current-spreading property of the LED device can be
sufficiently obtained by the current-spreading layer 8, such as ITO
or ZnO transparent conductive film, made of metal oxide and formed
on the contact layer 7. Thus, the buffer layer 6 is not always
necessary to have the current-spreading property. Even if the
buffer layer 6 has a thickness, e.g., 10 .mu.m, or more than the
suitable thickness, the output of the LED device cannot be
increased significantly since the current-spreading layer 8 is
predominant in current-spreading effect. On the contrary, a
disadvantage will arise that the manufacturing cost of the LED
device is increased by the increased thickness to raise the
production cost of the LED device. In consideration of this, it is
suitable in production aspect that the thickness of the buffer
layer 6 is set to be around the suitable thickness as described
above.
[0130] (2) Second, the ohmic contact layer contacting the
current-spreading layer made of metal oxide, e.g., the ITO film 8,
is necessary to be doped with a dopant at a very high concentration
so that it has a very high carrier concentration.
[0131] For example, in case of the contact layer 7 doped with Zn,
its crystal material has desirably an Al mixture ratio of 0 to 0.4,
i.e., GaAs to Al0.4GaAs. Its carrier concentration is suitably
1.times.10.sup.19/cm.sup.3 or more, which is further preferred as
it is higher.
[0132] The ITO film 8 belongs basically to n-type semiconductor.
The LED device is generally fabricated p-side up. Thus, the LED
device using the ITO film 8 as the current spreading layer has, in
conductivity type, a junction of n/p/n viewing from the substrate
side. Therefore, the LED device has a large potential barrier
generated at the interface between the ITO film 8 and the p-type
semiconductor layer, and the LED device must have generally a very
high operating voltage.
[0133] To solve this problem, the p-type semiconductor layer needs
to be a p-type semiconductor layer with a very high carrier
concentration. The reason why the contact layer 7 has a narrow
bandgap is that the high carrier concentration can be facilitated
by such a narrow bandgap. In connection with the high carrier
concentration of the contact layer 7, it is important that the
current-spreading layer, e.g., the ITO film 8, contacting the
contact layer 7 has a high carrier concentration, so as to reduce
the tunnel voltage. For the same reason as the contact layer 7, it
has preferably a carrier concentration of
7.times.10.sup.20/cm.sup.3 or more.
[0134] The transparent conductive film (i.e., the current-spreading
layer 8 in the embodiment) with a carrier concentration of
7.times.10.sup.20/cm.sup.3 or more is formed by a vacuum deposition
method or a sputtering method. It is known that especially, a DC
sputtering method with RF superposed thereon is effective to
produce a transparent conductive film with a very high carrier
concentration. The other methods such as a coating method using MOD
(metal organic deposition) solution or spray pyrolysis deposition
can be used. However, they are not desirable since it is difficult
to obtain a transparent conductive film with a high carrier
concentration by them and an adverse effect may arise during the
formation due to heat applied to its epitaxial wafer.
[0135] (3) Third, it is preferred that the contact layer 7 has a
thickness of 1 nm or more and 30 nm or less. This is because the
contact layer 7 has a bandgap to be an absorption layer or the like
to light emitted from the active layer 4, and the optical output
lowers as the thickness thereof increases.
[0136] FIG. 6 is a graph showing a relationship between a thickness
of the contact layer 7 and a transmittance at the LED emission
wavelength. As shown in FIG. 6, the (visible-light) transmittance
at the emission wavelength lowers as the thickness of the contact
layer 7 increases. Thus, it is preferred that the thickness of the
contact layer 7 is about 30 nm in upper limit to provide the LED
device with a high output. If the thickness of the contact layer 7
is less than 1 nm (i.e., several angstroms (.ANG.)), it is
difficult to have the tunnel junction between the ITO film 8 and
the contact layer 7. Therefore, the operating voltage is difficult
to lower and stabilize. Accordingly, it is preferred that the
contact layer 7 contacting the ITO film 8 has a thickness of 1 nm
to 30 nm.
[0137] (4) Fourth, it is preferred that the current spreading layer
8 comprising a metal oxide has a thickness in a range of .+-.30% of
d calculated by an expression of:
d=A.times..lamda..sub.p/(4.times.n), where A is a constant (A=1 or
3), .lamda..sub.p is an emission peak wavelength (nm) of the LED
device, and n is a refractive index.
[0138] The ITO film 8 exemplified as the current-spreading layer
formed on the LED epitaxial wafer has a refractive index nearly in
the middle of the semiconductor layer and the air layer, and it
functions optically as a reflection preventing film. Thus, in order
to increase the light extraction efficiency to enhance the optical
output of the LED device, it is preferred that it has a thickness
according to the above expression.
[0139] However, as the ITO film 8 increases in thickness, the
transmittance may degrade. If the intrinsic transmittance of the
ITO film 8 lowers, a rate of light absorbed by the ITO film 8 after
being emitted from the active layer 4 increases. As a result, the
optical output will lower. Further, as the current-spreading layer
8 increases in thickness, the optical interference in the current
spreading layer will increase and wavelength region with high light
extraction efficiency will be narrowed. In this regard, FIG. 7
shows the measurement result that, preparing a sample that the ITO
film 8 is formed on the GaAs substrate 1, spectra of reflected
light is measured when light is incident perpendicularly to this
sample.
[0140] For these reasons, the preferred thickness d of the current
spreading layer is determined by the expression, where the constant
A is preferably 1 or 3, most preferably A=1. The ITO film 8
exemplified as the current spreading layer formed on the LED
epitaxial wafer has desirably a thickness in the range of .+-.30%
of d calculated by the expression. This is because a wavelength
band, i.e., a wavelength band with high light extraction
efficiency, with an optically low reflectivity to serve as a
reflection preventing film has a certain width. For example,
tolerance in thickness of the reflection preventing film to have a
reflectivity of 15% or less when light is incident perpendicularly
to the LED epitaxial wafer is in the range of .+-.30% of d
calculated by the relational expression. If d exceeds .+-.30%, the
effect of the reflection preventing film lowers and the optical
output of the LED device lowers relatively.
[0141] (5) Fifth, the diffusion suppressing layer formed
neighboring the active layer 4 has preferably a thickness of 300 nm
or less when it is inserted between the active layer 4 and the
p-type cladding layer 5, and 200 nm or less when it is inserted
between the active layer 4 and the n-type cladding layer 3. The
reasons for the upper limits in thickness are as described
below.
[0142] With regard to the former upper-limit thickness (300 nm or
less), as intended by the invention, the dopant will not be
diffused and much penetrated into the active layer 4 since the
interdiffusion between the p-type dopants, Zn and Mg, can be
suppressed effectively by controlling suitably the H (hydrogen)
concentration and thickness of the buffer layer 6. However, if the
thickness of the buffer layer 6 is not set to have a safety margin,
or depending on an error in dopant concentration or thickness
during the epitaxial growth, the penetration of the dopant into the
active layer 4 may arise. Even in this case, by forming the
diffusion suppressing layer between the active layer 4 and the
p-type cladding layer 5 as well as controlling suitably the H
concentration and thickness of the buffer layer 6, the lifetime and
stability of the LED device can be enhanced. However, it is not
always suitable to increase simply the thickness of the diffusion
suppressing layer, and the thickness is to be under the upper limit
as described above. Namely, if the diffusion suppressing layer is
too thick, carrier (i.e., hole) cannot be efficiently injected from
the p-type cladding layer 5. Thereby, the forward voltage of the
LED device will be increased so that properties required for the
LED device will degrade. Thus, it is preferable that the diffusion
suppressing layer formed between the active layer 4 and the p-type
cladding layer 5 has suitably a thickness of 300 nm or less, more
suitably 200 nm or less.
[0143] With regard to the latter upper-limit thickness (200 nm or
less), for the same reason as the diffusion suppressing layer
formed on the side of the p-type cladding layer 5, the n-type
dopant doped in the n-type cladding layer 3 may be not a little
diffused into the active layer 4. Also, even though the diffusion
distance is short, the n-type dopant doped in the n-type cladding
layer 3 may be diffused into the active layer 4 by so-called memory
effect during the growth of the n-type cladding layer 3 and the
active layer 4. Due to the diffusion of the n-type dopant, the
optical output of the LED device will lower. To solve this problem,
it is desirable to form the diffusion suppressing layer between the
active layer 4 and the n-type cladding layer 3. The diffusion
suppressing layer has suitably the upper limit in thickness, i.e.,
200 nm or less, which is based on the same reasons as the diffusion
suppressing layer formed on the side of the p-type cladding layer
5, more suitably 100 nm or less.
[0144] (6) Sixth, it is preferred that the total number of pairs in
the light reflecting layer is in the range of 10 to 30 pairs. The
lower limit is set to be 10 because 10 pairs are needed to have a
sufficient reflectivity in the light reflecting layer. In this
regard, FIG. 8 shows the relationship between the number of pairs
in the light reflecting layer and the perpendicular
reflectivity.
[0145] The reasons for the upper limit are as follows.
[0146] Even if the light reflecting layer is thickened so much, the
reflectivity or the optical output of the LED device is not always
increased by that much. As shown in FIG. 8, the reflectivity of the
light reflecting layer tends to be almost saturated at 20 and a few
pairs or more, being completely saturated at 30 pairs or more.
Thus, the number of pairs needs to be more than a certain number in
order to have an effective reflectivity. In addition, in order to
fabricate the LED device and LED epitaxial wafer at low cost and
efficiently, it is preferred that the number of pairs in the light
reflecting layer has an upper limit.
[0147] According to the above reasons, the number of pairs is to be
selected in the range of 10 to 30 pairs, more preferably 15 to 25
pairs.
[0148] Materials suitable for the light reflecting layer can be
Al.sub.xGa.sub.1-xAs (0.4.ltoreq.X.ltoreq.1) or
(Al.sub.xGa.sub.1-x).sub.yIn.sub.1-yP (0.ltoreq.X.ltoreq.1 and
0.4.ltoreq.Y.ltoreq.0.6). The reason for selecting these materials
is that they are almost lattice-matched to the GaAs substrate 1 and
optically transparent to a wavelength of light emitted from the LED
device. It is known that, as a difference in refractive index
between two materials to compose the DBR, i.e., the light
reflecting layer, is increased, the reflection wavelength band of
light is broadened and the reflectivity is increased. Therefore, it
is preferred that the above materials are selected.
[0149] (7) Seventh, it is preferred that the buffer layer 6
comprises Al.sub.xGa.sub.1-xAs (0.4.ltoreq.X.ltoreq.1). The reason
for selecting this range is that the buffer layer 6 is located on
the side of the light extracting surface of the LED device, i.e.,
on the surface side of the LED device, and therefore it is
advantageous in optical output that it is transparent to light
emitted from the LED. If it comprises AlGaAs out of the range, it
is not preferable from the viewpoint of obtaining a high-output LED
device although the effects of the invention are not harmed.
[0150] (8) Eighth, it is preferred that the p-type cladding layer 5
has an Mg concentration in the range of 1.times.10.sup.17 to
5.times.10.sup.18/cm.sup.3. The reasons for specifying the lower
limit (1.times.10.sup.17/cm.sup.3) are that, if the Mg
concentration is less than the lower limit, the carrier
concentration of the layer 5 becomes so low that it is difficult to
have a sufficient effect as the carrier supply layer, whereby the
optical output of the LED device is reduced. The reasons for
specifying the upper limit (5.times.10.sup.18/cm.sup.3) are that,
if Mg is doped excessively, crystal defects will arise in the
p-type cladding layer 5 nearly according to the Mg concentration so
that the diffusion of the dopant is prompted to reduce the internal
quantum efficiency of the LED, thus causing a reduction in optical
output in the LED device.
EXAMPLES OF THE FIRST EMBODIMENT
[0151] The first embodiment of the invention will be further
detailed below in Examples 1-4 and Comparative Examples 1-2.
Example 1
[0152] In Example 1, a red LED epitaxial wafer with a structure as
shown in FIG. 1 and an emission wavelength of about 630 nm is
fabricated. Its epitaxial growth method, epitaxial layer thickness,
epitaxial layer structure, electrode formation method, and LED
device fabrication method will be explained below.
[0153] On the n-type GaAs substrate 1 doped with Si, by the MOVPE
method, the n-type (Si-doped) GaAs buffer layer 2 (with a thickness
of 200 nm and a carrier concentration of
1.times.10.sup.18/cm.sup.3), the n-type (Si-doped)
(Al.sub.0.7Ga.sub.0.3).sub.0.5In.sub.0.5P cladding layer 3 (with a
thickness of 400 nm and a carrier concentration of
1.times.10.sup.18/cm.sup.3), the undoped
(Al.sub.0.1Ga.sub.0.9).sub.0.5In.sub.0.5P active layer 4 (with a
thickness of 600 nm), the p-type (Mg-doped)
(Al.sub.0.7Ga.sub.0.3).sub.0.5In.sub.0.5P cladding layer 5 (with a
thickness of 400 nm and a carrier concentration of
1.2.times.10.sup.18/cm.sup.3), the p-type (Mg-doped)
(Al.sub.0.85Ga.sub.0.15)As buffer layer 6 (with a thickness of 5
.mu.m and a carrier concentration of 2.times.10.sup.18/cm.sup.3),
and the p-type (Zn-doped) Al.sub.0.1Ga.sub.0.9)As contact layer 7
(with a thickness of 3 nm and a carrier concentration of
7.7.times.10.sup.19/cm.sup.3) are sequentially grown in
lamination.
[0154] The growth temperature in the MOVPE growth is set at
650.degree. C. from the n-type GaAs buffer layer 2 to the p-type
buffer layer 6, and the growth temperature of the p-type contact
layer 7 is set at 550.degree. C. The other growth conditions are a
growth pressure of about 6666 Pa (50 Torr), a growth rate of the
respective layers in the range of 0.3 to 1.1 nm/sec, and a V/III
ratio of about 150. However, the p-type contact layer 7 has a V/III
ratio of 11. The p-type buffer layer 6 is made of AlGaAs with an Al
composition of about 0.8 to 0.9 and the V/III ratio during the
growth (in this case, about 50 in V/III ratio) is set such that the
H (hydrogen) concentration of the p-type buffer layer 6 is about
3.times.10.sup.18/cm.sup.3. Herein, the V/III ratio is a ratio
(quotient) obtained by a denominator, which is the number of moles
of a group III material such as TMGa or TMAl, and a numerator,
which is the number of moles of a group V material such as
AsH.sub.3 or PH.sub.3.
[0155] A material used in the MOVPE growth can be an organic metal
such as trimethyl gallium (TMGa) or triethyl gallium (TEGa) for Ga
source, trimethyl aluminum (TMAl) for Al source and trimethyl
indium (TMIn) for In source, or a hydride gas such as arsine
(AsH.sub.3) for As source and phosphine (PH.sub.3) for P source. A
dopant material for an n-type layer such as the n-type buffer layer
2 can be disilane (Si.sub.2H.sub.6). A dopant material for a
conductivity-determining impurity of a p-type layer such as the
p-type clad layer 5 and the p-type buffer layer 6 can be
biscyclopentadienyl magnesium (Cp.sub.2Mg). However, diethyl zinc
(DEZn) is used only for the p-type contact layer 7.
[0156] Furthermore, a dopant material for a
conductivity-determining impurity of an n-type layer can be also
hydrogen selenide (H.sub.2Se), monosilane (SiH.sub.4), diethyl
tellurium (DETe) and dimethyl tellurium (DMTe). A Zn source can be
also dimethyl zinc (DMZn).
[0157] Then, after the LED epitaxial wafer is taken out from the
MOVPE furnace, the ITO film 8 with a thickness of about 80 nm is
formed by the vacuum deposition method on the surface of the wafer,
i.e., the upper surface side of the p-type contact layer 7. In this
structure, the ITO film 8 composes the current spreading layer.
[0158] At this time, an evaluating glass substrate set in the same
batch as for the deposition of the ITO film 8 is taken out and cut
into a size enough for the Hall measurement, and the electric
characteristics of only the ITO film 8 are evaluated. As a result,
a carrier concentration is 1.15.times.10.sup.21/cm.sup.3, a
mobility is 16.3 cm.sup.2/Vs, and a resistivity is
3.32.times.10.sup.-4.OMEGA.cm.
[0159] Then, the surface electrode 9 as a circular electrode and
with a diameter of about 120 .mu.m is provided in the form of a dot
matrix on the upper surface of the epitaxial wafer by the vacuum
deposition method by using tools or materials and process used for
a general photolithography process such as a resist and a mask
aligner. A liftoff method is used in electrode formation after the
deposition. The surface electrode 9 is formed by sequentially
depositing Ni (nickel) and Au (gold) with thicknesses of 20 nm and
500 nm, respectively. Furthermore, on the entire bottom surface of
the epitaxial wafer, the back surface electrode 10 is formed by the
same vacuum deposition method. The back surface electrode 10 is
formed by sequentially depositing AuGe (gold-germanium alloy,
germanium content of 7.4%), Ni (nickel), and Au (gold) with
thicknesses of 60 nm, 10 nm, and 500 nm, respectively. Then, an
alloy process to alloy the electrode is performed such that the
electrode is heated at 440.degree. C. in a nitrogen gas atmosphere
for 5 minutes.
[0160] Then, the LED epitaxial wafer with the electrode formed as
described above is cut by a dicer such that the circular surface
electrode 9 is located at the center, so as to obtain an LED bear
chip with a chip size of 300 .mu.m square. Then, the LED bear chip
is mounted (die-bonding) through an Ag paste on a TO-18 stem. Then,
the LED bear chip is wire-bonded to have the LED device.
[0161] Initial characteristics of the LED device thus fabricated
are evaluated. As a result, the LED device has excellent initial
characteristics, i.e., an optical output of 1.01 mW and an
operating voltage of 1.842 V during the power feeding at 20 mA (in
evaluation).
[0162] Furthermore, a continuous current test for 168 hours (=1
week) is conducted where the LED device is driven at 50 mA in the
environment of room temperature (about 23.degree. C.) and
atmospheric humidity (about 40%). As relative values as compared to
those before the test, optical output is 102.2% (provided that
optical output before the power feeding is 100%; hereinafter
referred to as a relative output), and operation voltage is 1.843 V
(about 0.1% increased).
[0163] SIMS analysis is conducted for the LED device just after the
formation of the LED device and for the LED device after the
continuous current test under the above conditions.
[0164] FIG. 12 is a diagram showing the result of SIMS analysis
after the continuous current test. Meanwhile, the LED device sample
as used in FIG. 12 is prepared such that its surface is removed
several micrometers by mechanical polishing so as to enhance the
measurement resolution of the SIMS analysis.
[0165] As the result of the SIMS analysis, it is confirmed that the
H (hydrogen) concentration of the AlGaAs buffer layer 6 is about
3.times.10.sup.18/cm.sup.3 both before and after the continuous
current test, and that, in the LED device of Example 1 after the
continuous current test, Zn is not diffused into the active layer
4.
[0166] In consideration of the above results, the H concentration
of the buffer layer 6 is measured by the SIMS analysis while
varying the V/III ratio during the formation of the AlGaAs buffer
layer 6. FIG. 13A shows the measurement result. As shown in FIG.
13A, it is confirmed that the H concentration of the buffer layer 6
well correlates with the V/III ratio during the formation of the
AlGaAs buffer layer 6. However, the H concentration of the buffer
layer 6 is not determined simply by the V/III ratio, and it also
depends on the growth temperature or the Al composition of the
buffer layer 6. Thus, the control of the H concentration is not
limited only to the V/III ratio.
[0167] Further, using samples fabricated varying the V/III ratio as
shown in FIG. 13A, the diffusion length of Zn doped in the contact
layer 7 in the LED structure as described in Example 1 is measured.
The relationship between the H concentration of the AlGaAs buffer
layer and the Zn diffusion length is shown in FIG. 2A. FIG. 2A
shows that the Zn diffusion length depends on the H concentration
of the AlGaAs buffer layer. This result is represented by the
expression: L=6.869.times.10.sup.-15.times.N.sub.H.sup.0.788, where
N.sub.H is an H (hydrogen) concentration [cm.sup.-3] of the buffer
layer 6 and L is a diffusion length [.mu.m] of a dopant doped in
the p-type contact layer 7. When the AlGaAs buffer layer 6 has a
thickness of more than the diffusion length L obtained by the
expression, Zn doped in the contact layer 7 is not diffused into
the active layer 4 so that the LED device can be excellent both in
initial property and reliability. Thus, the excellent properties of
the LED device in Example 1 are based on the above reasons.
Further, almost all of the LED devices in Example 1 can be
fabricated without being broken.
Example 2
[0168] In Example 2, a red LED epitaxial wafer with a structure as
shown in FIG. 3 and an emission wavelength of about 630 nm is
fabricated. Its epitaxial growth method, epitaxial layer thickness,
epitaxial layer structure, electrode formation method, and LED
device fabrication method are basically the same as those in
Example 1 (FIG. 1). Different points from Example 1 will be
described below.
[0169] Example 2 has the feature that a device structure is
employed that a semiconductor layer not positively doped, so-called
undoped layer, is formed as the diffusion suppressing layer 11
between the active layer 4 and the p-type cladding layer 5. The
diffusion suppressing layer 11 serves to prevent the p-type dopant
doped into the upper p-type semiconductor layer including the
p-type cladding layer 5 from penetrating into the active layer 4.
The layer 11 has the same composition as the p-type cladding layer
5, and a thickness of 100 nm.
[0170] In addition, as a reference example for Example 2, an LED
epitaxial wafer is fabricated such that the diffusion suppressing
layer 11 is similarly inserted into an LED device as described
later in Comparative Example 1 (FIG. 9).
[0171] Then, the LED epitaxial wafers thus fabricated are formed
into devices as in Example 1.
[0172] Initial characteristics of the LED device thus fabricated
are evaluated. As a result, the LED device in the reference example
has excellent initial characteristics, i.e., an optical output of
0.96 mW and an operating voltage of 1.854 V during the power
feeding at 20 mA (in evaluation)
[0173] Further, the continuous current test is conducted on the
same conditions as Example 1. As a result, the LED device in the
reference example has a relative output of 75.3% and an operating
voltage of 1.903 V (about 2.3% increased), where the relative
output is enhanced as compared to Comparative Example 1 described
later.
[0174] Then, the LED device in Example 2, i.e., the LED device with
the diffusion suppressing layer 11 added to the LED structure in
Example 1 as shown in FIG. 3, is evaluated. As a result, the LED
device in Example 2 has excellent initial characteristics, i.e., an
optical output of 0.98 mW and an operating voltage of 1.843 V
during the power feeding at 20 mA (in evaluation).
[0175] Further, for the LED device in Example 2, the continuous
current test is conducted on the same conditions as Example 1. As a
result, the LED device has a relative output of 102.1% and an
operating voltage of 1.844 V (about 0.1% increased).
[0176] As described above, although the diffusion suppressing layer
11 used in Example 2 does not completely suppressing the diffusion
of Zn, the diffusion suppressing layer 11 can serve to reduce the
amount of Zn being diffused into the active layer 4. As a result,
the LED device in the reference example can have the relative
output enhanced as compared to Comparative Example 1 described
later. Thus, even when the invention is applied to a device
structure not subjected to the diffusion of Zn, the diffusion
suppressing layer 11 can offer the similar effect without causing
any adverse effect.
Example 3
[0177] In Example 3, a red LED epitaxial wafer with a structure as
shown in FIG. 4 and an emission wavelength of about 630 nm is
fabricated. Its epitaxial growth method, epitaxial layer thickness,
epitaxial layer structure, electrode formation method, and LED
device fabrication method are basically the same as those in
Example 1 (FIG. 1).
[0178] Example 3 has the feature that a light reflecting layer 12
is formed between the n-type GaAs buffer layer 2 and the n-type
cladding layer 3 such that it comprises 20 pairs of DBR
(distributed Bragg reflector) where an n-type AlInP layers and an
n-type Al.sub.0.4Ga.sub.0.6As layer, 20 layers respectively, are
alternately formed.
[0179] The light reflecting layer 12 has a thickness obtained by
.lamda..sub.p/(4.times.n) where .lamda..sub.p is an emission peak
wavelength (nm) of the LED device and n is a refractive index of
the semiconductor material to compose the light reflecting layer
12. The light reflecting layer 12 has uniformly a carrier
concentration of about 1.times.10.sup.18/cm.sup.3.
[0180] Initial characteristics of the LED device thus fabricated
are evaluated. As a result, the LED device in Example 3 has
excellent initial characteristics, i.e., an optical output of 1.57
mW and an operating voltage of 1.853 V during the power feeding at
20 mA (in evaluation). Further, as the result of the continuous
current test, the LED device in Example 3 has a relative output of
102.0% and an operating voltage of 1.855 V (about 0.1%
increased).
[0181] As described above, in addition to the effects of Example 1,
the LED device in Example 3 can have an optical output higher than
that of Example 1 by providing the light reflecting layer 12 of
semiconductor multilayer between the n-type buffer layer 2 and the
n-type cladding layer 3. This effect is produced by the effective
light extraction efficiency enhanced by the light reflecting layer
12.
Example 4
[0182] In Example 4, a red LED epitaxial wafer with a structure as
shown in FIG. 5 and an emission wavelength of about 630 nm is
fabricated. Its epitaxial growth method, epitaxial layer thickness,
epitaxial layer structure, electrode formation method, and LED
device fabrication method are basically the same as those in
Example 1 (FIG. 1).
[0183] Example 4 has the feature that a MQW (multiquantum well)
active layer 13 is used instead of the active layer 4. The MQW is
composed of 40.5 pairs of a
(Al.sub.0.5Ga.sub.0.5).sub.0.5In.sub.0.5P (about 7.5 nm thick)
barrier layer and a Ga.sub.0.5In.sub.0.5P (about 5.5 nm thick) well
layer.
[0184] A modification of Example 4 is an LED (not shown) that the
composition ratio of Ga and In in the well layer of the MQW active
layer 13 is changed to have a strained multiquantum well structure
so that a compressive or tensile stress is applied to a start GaAs
substrate. The well layer with the strained multiquantum well
structure in the modification is composed such that the Ga
composition is reduced and the In composition is increased by that
difference, and it is subjected to a compressive strain caused by a
lattice mismatch that the lattice constant of the well layer is
different from an underlying layer such as the n-type cladding
layer 3.
[0185] The two LED devices (Example 4 and the modification) thus
fabricated respectively have excellent initial characteristics,
i.e., optical outputs of 1.16 and 1.27 mW and operating voltages of
1.843 and 1.844 V during power feeding at 20 mA (in
evaluation).
[0186] Further, when the continuous current test is conducted under
the same conditions as Example 1, the two LED devices have relative
outputs of 101.9% and 100.9%, respectively.
[0187] As described above, in Example 4 and the modification each
employing the MQW and strained multiquantum well structure instead
of the active layer 4 of Example 1, the optical output is increases
as compared to Example 1. Thus, by employing the quantum well
structure, the internal quantum efficiency of the LED device can be
increased to enhance the total characteristics of the LED device.
Further, the features of Example 1 can be applied to the quantum
well structure of Example 4 and the modification.
[0188] In Examples 1-4 of the invention, the red LED device with an
emission wavelength of 630 nm is fabricated. However, other LED
devices made of the same AlGaInP-based material with an emission
wavelength of 560 to 660 nm can also have the effects of the
invention by setting suitably the H (hydrogen) concentration and
the thickness of the buffer layer.
[0189] In Examples 1-4 of the invention, the LED device structure
is employed that the buffer layer 2 is formed between the GaAs
substrate land the n-type cladding layer 3. However, even when the
n-type cladding layer 3 is formed directly on the GaAs substrate 1,
the effects of the invention can be obtained.
[0190] Although Examples 1-4 have the circular surface electrode 9,
the other electrode shape such as rectangular, rhombic and
polygonal, and further the electrode shape being accompanied with a
wing-shaped or branch-shaped portion can be used. Electrodes with
such a shape can also have the effects of the invention.
[0191] In Examples 1-4, the semiconductor substrate comprises GaAs.
Alternatively, an LED epitaxial wafer may comprise a Ge substrate,
or GaAs or Ge substrate as a start substrate and then replaced by
Si or a metallic self-standing substrate with a higher thermal
conductivity than Si. Thereby, the effects of the invention can be
obtained.
[0192] In Examples 1-4, the buffer layer 6 comprises AlGaAs which
has an Al composition in the range of about 0.8 to 0.9. However,
the invention is not limited to the Al composition. Even when the
Al composition of the AlGaAs buffer layer 6 is not in the above
range, or even when the V/III ratio is not in the range of Examples
1-4, there is no problem if only the H (hydrogen) concentration and
the thickness of the buffer layer 6 are suitably set according to
the invention.
[0193] For example, in case of an LED device with an emission
wavelength of about 650 nm, even when the Al composition of the
AlGaAs buffer layer 6 is in the range of about 0.6 to 0.7, light
emitted from the active layer is little absorbed so that the LED
device can have a high optical output. In addition, since the H
concentration of the buffer layer 6 decreases directly in
connection with the reduction in Al composition, the V/III ratio
can be set lower than Examples 1-4 using the buffer layer 6 with an
Al composition of 0.8 to 0.9.
[0194] In case of an LED device with an emission wavelength of
about 570 nm, in order to eliminate the optical absorption to be
caused by the buffer layer 6, the Al composition of the buffer
layer 6 can be set to be around 0.9 so that the V/III ratio can be
higher than the conventional one. Thus, even when the fabrication
parameters such as a composition of material composing the buffer
layer, a V/III ratio during the epitaxial growth and a growth
temperature are changed, the effects of the invention can be
obtained by satisfying the requirements of the invention.
[0195] In Examples 1-4, the current-spreading layer 8 comprises
ITO. Instead of the ITO, a typical transparent conductive film such
as ZnO and CTO film with a high transmittance for visible light and
a low electrical resistivity can be used as the current-spreading
layer 8. However, a more important factor for a material used as
the current-spreading layer than the above two factors is
enhancement in carrier concentration. Since the importance of the
carrier concentration is as described above, materials applicable
to the current-spreading layer will be limited to some extent in
consideration of a reduction in the operating voltage of the LED
device. Thus, a suitable material should be selected from the
materials.
Comparative Example 1
[0196] In Comparative Example 1, a red LED epitaxial wafer with a
structure as shown in FIG. 9 and an emission wavelength of about
630 nm is fabricated. Its epitaxial growth method, epitaxial layer
thickness, epitaxial layer structure, electrode formation method,
and LED device fabrication method are basically the same as those
in Example 1 (FIG. 1). Different points from Example 1 will be
described below.
[0197] Comparative Example 1 is prepared such that the V/III ratio
during the growth of a p-type buffer layer 16 is 11 and the H
concentration of the p-type buffer layer 16 is
2.3.times.10.sup.19/cm.sup.3.
[0198] Then, the LED epitaxial wafer thus fabricated is formed into
devices as in Example 1.
[0199] Initial characteristics of the LED device thus fabricated
are evaluated. As a result, the LED device in Comparative Example 1
has initial characteristics, i.e., an optical output of 0.90 mW and
an operating voltage of 1.856 V during the power feeding at 20 mA
(in evaluation).
[0200] Further, the continuous current test is conducted on the
same conditions as Example 1. As a result, the LED device in
Comparative Example 1 has a relative output of 54% and an operating
voltage of 1.916 V (about 3% increased).
[0201] SIMS analysis is conducted for the LED device just after the
formation of the LED device and for the LED device after the
continuous current test under the above conditions.
[0202] FIG. 11 is a diagram showing the result of SIMS analysis
after the continuous current test. Meanwhile, the LED device sample
as used in FIG. 11 is prepared such that its surface is removed
several micrometers by mechanical polishing so as to enhance the
measurement resolution of the SIMS analysis.
[0203] As the result of the SIMS analysis, it is confirmed that the
H (hydrogen) concentration of the buffer layer 16 is
2.3.times.10.sup.19/cm.sup.3 both before and after the continuous
current test, and that, in the LED device of Comparative Example 1
after the continuous current test, Zn doped as a dopant into the
p-type contact layer 7 is diffused into the active layer 4. Thus,
deterioration in lifetime, i.e., reliability in the LED device of
Comparative Example 1 is caused by the dopant diffusion into the
active layer.
Comparative Example 2
[0204] In Comparative Example 2, a red LED epitaxial wafer with a
structure as shown in FIG. 10 and an emission wavelength of about
630 nm is fabricated. Its epitaxial growth method, epitaxial layer
thickness, epitaxial layer structure, electrode formation method,
and LED device fabrication method are basically the same as those
in Comparative Example 1 (FIG. 9) Different points from Comparative
Example 1 will be described below.
[0205] Comparative Example 2 is not formed with the p-type buffer
layer 16. The p-type cladding layer 5 has a thickness of about 400
nm, which is enough to have a carrier confining effect and to serve
as a carrier (hole) supply layer. Namely, the p-type cladding layer
5 with a thickness of about 400 nm can sufficiently serve as a
cladding layer. Thus, the LED device of Comparative example 2 has
the same structure as Comparative Example 1 except not having the
AlGaAs buffer layer 16.
[0206] Then, the LED epitaxial wafer thus fabricated is formed into
devices as in Comparative Example 1.
[0207] Initial characteristics of the LED device thus fabricated
are evaluated. As a result, the LED device in Comparative Example 2
has initial characteristics, i.e., an optical output of 0.88 mW and
an operating voltage of 1.843 V during the power feeding at 20 mA
(in evaluation).
[0208] However, in evaluating the initial characteristics, about
20-30% of the devices are broken so that it does not emit light.
Although the device not broken has the abovementioned
characteristics, the about 20-30% of the devices broken cannot be
evaluated. This is assumed because the device is broken in the wire
bonding process before the device evaluation. When the continuous
current test is conducted for the devices not broken under the same
conditions as Comparative Example 1, the LED device has a relative
output of 71% and an operating voltage of 1.853 V (about 0.5%
increased).
[0209] As described above, in case of the structure without the
buffer layer, the product yield deteriorates and the optical output
and reliability is not sufficient. Namely, although the relative
output is a little improved as compared to Comparative Example 1,
the product yield is contrary reduced (In Comparative Example 1,
none of the LED devices is broken).
Second Embodiment
[0210] FIG. 1 is a cross sectional view showing an AlGaInP-based
red LED in the second preferred embodiment of the invention.
[0211] The LED comprises, sequentially formed on the n-type GaAs
substrate 1 as a semiconductor substrate, the n-type GaAs buffer
layer 2, the n-type AlGaInP cladding layer 3 (herein also simply
called n-type cladding layer), the undoped AlGaInP active layer 4
(herein also simply called active layer) and the p-type AlGaInP
cladding layer 5 (herein also simply called p-type cladding layer
5), where the layers 3-5 are epitaxially grown to compose a
light-emitting portion. Further, the p-type (As-based) AlGaAs
contact layer 7 (herein also simply called p-type contact layer 7)
doped with a p-type dopant at a high concentration is formed on the
p-type cladding layer 5. Further, the ITO film (current-spreading
layer) 8, which is a current-spreading layer made of a metal oxide
and a transparent conductive film, is formed on the p-type contact
layer 7. The surface electrode 9 is formed on the surface of the
ITO film 8, and the back surface electrode 10 is formed on the back
surface of the n-type GaAs substrate 1.
[0212] The p-type contact layer 7 is made of Al.sub.xGa.sub.1-xAS
(wherein 0.ltoreq.x.ltoreq.0.4), and has a thickness of 1 nm or
more and 30 nm or less. Zn as a p-type dopant is doped at a high
carrier concentration of 1.times.10.sup.19/cm.sup.3 or more.
[0213] The film thickness of the ITO film 8 as the current
spreading layer is in a range of .+-.30% of d calculated by a
relational expression of d=A.times..lamda..sub.p/(4.times.n), where
A is a constant (A=1 or 3), .lamda..sub.p is an emission wavelength
(nm), and n is a refractive index. The ITO film 8 as the current
spreading layer is formed by a vacuum deposition method or a
sputtering method. The ITO film 8 has preferably a carrier
concentration of 7.times.10.sup.20/cm.sup.3 or more just after the
film formation.
[0214] A feature of the LED is that the p-type buffer layer 6 is
formed between the p-type contact layer 7 and the p-type cladding
layer 5, and it comprises a group III/V semiconductor doped with Mg
as a p-dopant.
[0215] The p-type buffer layer 6 is formed of an
Al.sub.xGa.sub.1-xAs (0.4.ltoreq.x.ltoreq.1) which is optically
transparent to the emission wavelength and lattice-matched to the
AlGaInP-based material to compose the light-emitting portion. The
p-type buffer layer 6 has a thickness t of a diffusion length or
more of Zn doped in the p-type contact layer 7.
[0216] The p-type buffer layer 6 is an AlGaAs layer with a high Al
composition, optically transparent to the emission wavelength of an
LED device composed of an AlGaInP-based material, easier to grow
its crystal than a four-element material such as AlGaInP, and
almost lattice-matched to the AlGaInP-based material to compose the
light-emitting portion. Thus, the p-type buffer layer 6 is formed
of the material capable of lowering the operating voltage of the
LED device.
[0217] The diffusion length of Zn doped in the p-type contact layer
7 has, as shown in FIG. 2B, a correlation with a concentration of C
(carbon) contained in the p-type buffer layer 6. The Zn diffusion
length is represented by an expression:
L=6.872.times.10.sup.-14.times.N.sub.C.sup.0.733, where N.sub.C is
a C concentration [cm.sup.-3] of the buffer layer and L is a
diffusion length [.mu.m] of a dopant doped in the p-type contact
layer 7.
[0218] In this embodiment, the C concentration in the p-type buffer
layer 6 is set such that the thickness t of the p-type buffer layer
6 is greater than the Zn diffusion length L obtained by the above
expression or from the curve in FIG. 2B. The setting of C
concentration is closely controlled in consideration of parameters
such as the V/III ratio during the growth of the p-type buffer
layer 6 or Al composition of the p-type buffer layer.
[0219] For example, when the p-type buffer layer 6 has a thickness
of 5 .mu.m, the C concentration in the p-type buffer layer 6 is set
to be 1.times.10.sup.18/cm.sup.3. The diffusion length of Zn doped
in the p-type contact layer 7 is obtained by the above expression
or from the curve in FIG. 2B to be about 1.1 .mu.m, which falls
within the thickness of the p-type buffer layer 6. Thereby, the Zn
diffusion from the p-type contact layer 7 can be prevented, and,
therefore, the interdiffusion between Zn of the p-type contact
layer 7 and Mg of the other p-type semiconductor layers 5, 6 can be
effectively suppressed.
[0220] The inventors found that the interdiffusion reaction between
Zn of the p-type contact layer 7 and Mg of the p-type cladding
layer 5 and the p-type buffer layer 6 is in close relation with the
C concentration in the p-type buffer layer 6, and that this
phenomenon is significant especially when the p-type buffer layer 6
is an AlGaAs layer with a high Al composition. Therefore, since the
C concentration of the p-type buffer layer 6 can be set by
controlling closely the parameters such as the V/III ratio during
the growth of the p-type buffer layer 6 or Al composition of the
buffer layer 6, the thickness of the p-type buffer layer 6 can be
suitably set so as to control the interdiffusion distance between
Zn and Mg, and a long lifetime LED device can be obtained.
[0221] In this embodiment, between the n-type GaAs substrate 1 and
the n-type cladding layer 3, e.g., between the n-type GaAs buffer
layer 3 and the n-type cladding layer 3, a light reflecting layer
may be formed which comprises 10 pairs or more and 30 pairs or less
of two semiconductor layers different in refractive index, made of
high-refractive index material and low-refractive index material,
respectively.
[0222] Further, in this embodiment, between the active layer 4 and
the p-type cladding layer 5 or between the active layer 4 and the
n-type cladding layer 3, a diffusion-suppressing layer may be
formed which comprises any one or a combination of an undoped
semiconductor layer, a semiconductor layer with a lower dopant
concentration than the p-type cladding layer 5, and a semiconductor
layer doped with an n-type dopant and a p-type dopant
simultaneously to be neutral in pseudo conduction type. The
diffusion-suppressing layer has desirably a thickness of 300 nm or
less when it is inserted between the active layer 4 and the p-type
cladding layer 5. Alternatively, it has desirably a thickness of
200 nm or less when it is inserted between the active layer 4 and
the n-type cladding layer 3.
[0223] The reasons for employing the LED structure as described in
this embodiment will be explained below.
[0224] (1) First, the thickness of the AlGaAs-based buffer layer 6
of the embodiment is set based on that it needs to prevent the
impurity in the p-type semiconductor from penetrating into the
active layer 4 due to the interdiffusion between Zn doped in the
p-type contact layer 7 and Mg doped in the other p-type
semiconductor layers 5, 6 so as to get an LED device with a long
lifetime and a high reliability.
[0225] The inventors found that the interdiffusion between Zn and
Mg is in close relation with the concentration of C contained in
the p-type buffer layer 6, and that the diffusion length (or
diffusion distance) of the impurities can be controlled by the
concentration of C contained in the p-type buffer layer 6. This can
be known by the experimental result to indicate the relationship
between the C concentration in the p-type buffer layer 6 and the
diffusion length of Zn as shown in FIG. 2B. Based on this result,
the diffusion length of Zn doped in the p-type contact layer 7 can
be obtained by the expression:
L=6.872.times.10.sup.-14.times.N.sub.C.sup.0.733, where N.sub.C is
a C concentration [cm.sup.-3] of the buffer layer 6 and L is a
diffusion length [.mu.m] of a dopant doped in the p-type contact
layer 7. Thus, in the embodiment, since the AlGaAs buffer layer 6
has a thickness t equal to or more than the Zn diffusion length L
obtained by the expression, the LED device can achieve a good
property both in initial characteristics and reliabilities.
[0226] Meanwhile, it is not always necessary to specify the upper
limit of the thickness t of the buffer layer 6. In other words, as
the thickness is increased from the suitable thickness (i.e., the
thickness t), the following situations will occur.
[0227] The current-spreading property of the LED device can be
sufficiently obtained by the current-spreading layer 8, such as ITO
or ZnO transparent conductive film, made of metal oxide and formed
on the contact layer 7. Thus, the buffer layer 6 is not always
necessary to have the current-spreading property. Even if the
buffer layer 6 has a thickness, e.g., 10 .mu.m, or more than the
suitable thickness, the output of the LED device cannot be
increased significantly since the current-spreading layer 8 is
predominant in current-spreading effect. On the contrary, a
disadvantage will arise that the manufacturing cost of the LED
device is increased by the increased thickness to raise the
production cost of the LED device. In consideration of this, it is
suitable in production aspect that the thickness of the buffer
layer 6 is set to be around the suitable thickness as described
above.
[0228] (2) Second, the ohmic contact layer 7 contacting the
current-spreading layer made of metal oxide, e.g., the ITO film 8,
is necessary to be doped with a dopant at a very high concentration
so that it has a very high carrier concentration.
[0229] For example, in case of the contact layer 7 doped with Zn,
its crystal material has desirably an Al mixture ratio of 0 to 0.4,
i.e., GaAs to Al0.4GaAs. Its carrier concentration is suitably
1.times.10.sup.19/cm.sup.3 or more, which is further preferred as
it is higher.
[0230] The ITO film 8 belongs basically to n-type semiconductor.
The LED device is generally fabricated p-side up. Thus, the LED
device using the ITO film 8 as the current spreading layer has, in
conductivity type, a junction of n/p/n viewing from the substrate
side. Therefore, the LED device has a large potential barrier
generated at the interface between the ITO film 8 and the p-type
semiconductor layer, and the LED device must have generally a very
high operating voltage.
[0231] To solve this problem, the p-type semiconductor layer needs
to be a p-type semiconductor layer with a very high carrier
concentration. The reason why the contact layer 7 has a narrow
bandgap is that the high carrier concentration can be facilitated
by such a narrow bandgap. In connection with the high carrier
concentration of the contact layer 7, it is important that the
current-spreading layer, e.g., the ITO film 8, contacting the
contact layer 7 has a high carrier concentration, so as to reduce
the tunnel voltage. For the same reason as the contact layer 7, it
has preferably a carrier concentration of
7.times.10.sup.20/cm.sup.3 or more.
[0232] The transparent conductive film (i.e., the current-spreading
layer 8 in the embodiment) with a carrier concentration of
7.times.10.sup.20/cm.sup.3 or more is formed by a vacuum deposition
method or a sputtering method. It is known that especially, a DC
sputtering method with RF superposed thereon is effective to
produce a transparent conductive film with a very high carrier
concentration. The other methods such as a coating method using MOD
(metal organic deposition) solution or spray pyrolysis deposition
can be used. However, they are not desirable since it is difficult
to obtain a transparent conductive film with a high carrier
concentration by them and an adverse effect may arise during the
formation due to heat applied to its epitaxial wafer.
[0233] (3) Third, it is preferred that the contact layer 7 has a
thickness of 1 nm or more and 30 nm or less. This is because the
contact layer 7 has a bandgap to be an absorption layer or the like
to light emitted from the active layer 4, and the optical output
lowers as the thickness thereof increases.
[0234] FIG. 6 is a graph showing a relationship between a thickness
of the contact layer 7 and a transmittance at the LED emission
wavelength. As shown in FIG. 6, the (visible-light) transmittance
at the emission wavelength lowers as the thickness of the contact
layer 7 increases. Thus, it is preferred that the thickness of the
contact layer 7 is about 30 nm in upper limit to provide the LED
device with a high output. If the thickness of the contact layer 7
is less than 1 nm (i.e., several angstroms (.ANG.)), it is
difficult to have the tunnel junction between the ITO film 8 and
the contact layer 7. Therefore, the operating voltage is difficult
to lower and stabilize. Accordingly, it is preferred that the
contact layer 7 contacting the ITO film 8 has a thickness of 1 nm
to 30 nm.
[0235] (4) Fourth, it is preferred that the current spreading layer
8 comprising a metal oxide has a thickness in a range of .+-.30% of
d calculated by an expression of:
d=A.times..lamda..sub.p/(4.times.n), where A is a constant (A=1 or
3), .lamda..sub.p is an emission peak wavelength (nm) of the LED
device, and n is a refractive index.
[0236] The ITO film 8 exemplified as the current-spreading layer
formed on the LED epitaxial wafer has a refractive index nearly in
the middle of the semiconductor layer and the air layer, and it
functions optically as a reflection suppressing film. Thus, in
order to increase the light extraction efficiency to enhance the
optical output of the LED device, it is preferred that it has a
thickness according to the above expression.
[0237] However, as the ITO film 8 increases in thickness, the
transmittance may degrade. If the intrinsic transmittance of the
ITO film 8 lowers, a rate of light to be absorbed by the ITO film 8
after being emitted from the active layer 4 increases. As a result,
the optical output will lower. Further, as the current-spreading
layer 8 increases in thickness, the optical interference will
increase in the current spreading layer and wavelength region with
high light extraction efficiency will be narrowed. In this regard,
FIG. 7 shows the measurement result that, preparing a sample that
the ITO film 8 is formed on the GaAs substrate 1, spectra of
reflected light is measured when light is incident perpendicularly
to this sample.
[0238] For these reasons, the preferred thickness d of the current
spreading layer is determined by the expression, where the constant
A is preferably 1 or 3, most preferably A=1. The ITO film 8
exemplified as the current spreading layer formed on the LED
epitaxial wafer has desirably a thickness in the range of .+-.30%
of d calculated by the expression. This is because a wavelength
band, i.e., a wavelength band with high light extraction
efficiency, with an optically low reflectivity to serve as a
reflection suppressing film has a certain width. For example,
tolerance in thickness of the reflection suppressing film to have a
reflectivity of 15% or less when light is incident perpendicularly
to the LED epitaxial wafer is in the range of .+-.30% of d
calculated by the relational expression. If d exceeds .+-.30%, the
effect of the reflection suppressing film lowers and the optical
output of the LED device lowers relatively.
[0239] (5) Fifth, the diffusion suppressing layer formed
neighboring the active layer 4 has preferably a thickness of 300 nm
or less when it is inserted between the active layer 4 and the
p-type cladding layer 5, and 200 nm or less when it is inserted
between the active layer 4 and the n-type cladding layer 3. The
reasons for the upper limits in thickness are as described
below.
[0240] With regard to the former upper-limit thickness (300 nm or
less), as intended by the invention, the dopant will not be
diffused and much penetrated into the active layer 4 since the
interdiffusion between the p-type dopants, Zn and Mg, can be
suppressed effectively by controlling suitably the C (carbon)
concentration and thickness of the buffer layer 6. However, if the
thickness of the buffer layer 6 is not set to have a safety margin,
or depending on an error in dopant concentration or thickness
during the epitaxial growth, the penetration of the dopant into the
active layer 4 may arise. Even in this case, by forming the
diffusion suppressing layer between the active layer 4 and the
p-type cladding layer 5 as well as controlling suitably the C
concentration and thickness of the buffer layer 6, the lifetime and
stability of the LED device can be enhanced. However, it is not
always suitable to increase simply the thickness of the diffusion
suppressing layer, and the thickness is to be under the upper limit
as described above. Namely, if the diffusion suppressing layer is
too thick, carrier (i.e., hole) cannot be efficiently injected from
the p-type cladding layer 5. Thereby, the forward voltage of the
LED device will be increased so that properties required for the
LED device will degrade. Thus, it is preferable that the diffusion
suppressing layer formed between the active layer 4 and the p-type
cladding layer 5 has suitably a thickness of 300 nm or less, more
suitably 200 nm or less.
[0241] With regard to the latter upper-limit thickness (200 nm or
less), for the same reason as the diffusion suppressing layer
formed on the side of the p-type cladding layer 5, the n-type
dopant doped in the n-type cladding layer 3 may be not a little
diffused into the active layer 4. Also, even though the diffusion
distance is short, the n-type dopant doped in the n-type cladding
layer 3 may be diffused into the active layer 4 by so-called memory
effect during the growth of the n-type cladding layer 3 and the
active layer 4. Due to the diffusion of the impurity, the optical
output of the LED device will lower. To solve this problem, it is
desirable to form the diffusion suppressing layer between the
active layer 4 and the n-type cladding layer 3. The diffusion
suppressing layer has suitably the upper limit in thickness, i.e.,
200 nm or less, which is based on the same reasons as the diffusion
suppressing layer formed on the side of the p-type cladding layer
5, more suitably 100 nm or less.
[0242] (6) Sixth, it is preferred that the total number of pairs in
the light reflecting layer is in the range of 10 to 30 pairs. The
lower limit is set to be 10 because 10 pairs are needed to have a
sufficient reflectivity in the light reflecting layer. In this
regard, FIG. 8 shows the relationship between the number of pairs
in the light reflecting layer and the perpendicular
reflectivity.
[0243] The reasons for the upper limit are as follows.
[0244] Even if the light reflecting layer is thickened so much, the
reflectivity or the optical output of the LED device is not always
increased by that much. As shown in FIG. 8, the reflectivity of the
light reflecting layer tends to be almost saturated at 20 and a few
pairs or more, being completely saturated at 30 pairs or more.
Thus, the number of pairs needs to be more than a certain number in
order to have an effective reflectivity. In addition, in order to
fabricate the LED device and LED epitaxial wafer at low cost and
efficiently, it is preferred that the number of pairs in the light
reflecting layer has an upper limit.
[0245] According to the above reasons, the number of pairs is to be
selected in the range of 10 to 30 pairs, more preferably 15 to 25
pairs.
[0246] Materials suitable for the light reflecting layer can be
Al.sub.xGa.sub.1-xAs (0.4.ltoreq.X.ltoreq.1) or
(Al.sub.xGa.sub.1-x).sub.yIn.sub.1-yP (0.ltoreq.X.ltoreq.1 and
0.4.ltoreq.Y.ltoreq.0.6). The reason for selecting these materials
is that they are almost lattice-matched to the GaAs substrate 1 and
optically transparent to a wavelength of light emitted from the
active layer. It is known that, as a difference in refractive index
between two materials to compose the DBR, i.e., the light
reflecting layer, is increased, the reflection wavelength band of
light is broadened and the reflectivity is increased. Therefore, it
is preferred that the above materials are selected.
[0247] (7) Seventh, it is preferred that the buffer layer 6
comprises Al.sub.xGa.sub.1-xAs (0.4.ltoreq.X.ltoreq.1). The reason
for selecting this range is that the buffer layer 6 is located on
the side of the light extracting surface of the LED device, i.e.,
on the surface side of the LED device, and therefore it is
advantageous in optical output that it is transparent to light
emitted from the LED. If it comprises AlGaAs out of the range, it
is not preferable from the viewpoint of obtaining a high-output LED
device although the effects of the invention are not harmed.
[0248] (8) Eighth, it is preferred that the p-type cladding layer 5
has an Mg concentration in the range of 1.times.10.sup.17 to
5.times.10.sup.18/cm.sup.3. The reasons for specifying the lower
limit (1.times.10.sup.17/cm.sup.3) are that, if the Mg
concentration is less than the lower limit, the carrier
concentration of the layer 5 becomes too low to have a sufficient
effect as the carrier supply layer, whereby the optical output of
the LED device is reduced. The reasons for specifying the upper
limit (5.times.10.sup.18/cm.sup.3) are that, if the Mg is doped
excessively, crystal defects will arise in the p-type cladding
layer 5 nearly according to the Mg concentration so that the
diffusion of the dopant is prompted to reduce the internal quantum
efficiency of the LED, thus the optical output of the LED device
lowers.
EXAMPLES OF THE SECOND EMBODIMENT
[0249] The second embodiment of the invention will be further
detailed below in Examples 5-8 and Comparative Examples 3-4.
Example 5
[0250] In Example 5, a red LED epitaxial wafer with a structure as
shown in FIG. 1 and an emission wavelength of about 630 nm is
fabricated. Its epitaxial growth method, epitaxial layer thickness,
epitaxial layer structure, electrode formation method, and LED
device fabrication method will be explained below.
[0251] On the n-type GaAs substrate 1 doped with Si, by using the
MOVPE method, the n-type (Si-doped) GaAs buffer layer 2 (with a
thickness of 200 nm and a carrier concentration of
1.times.10.sup.18/cm.sup.3), the n-type (Si-doped)
(Al.sub.0.7Ga.sub.0.3).sub.0.5In.sub.0.5P cladding layer 3 (with a
thickness of 400 nm and a carrier concentration of
1.times.10.sup.18/cm.sup.3), the undoped
(Al.sub.0.1Ga.sub.0.9).sub.0.5In.sub.0.5P active layer 4 (with a
thickness of 600 nm), the p-type
[0252] (Mg-doped) (Al.sub.0.7Ga.sub.0.3).sub.0.5In.sub.0.5P
cladding layer 5 (with a thickness of 400 nm and a carrier
concentration of 1.2.times.10.sup.18/cm.sup.3), the p-type
(Mg-doped) (Al.sub.0.85Ga.sub.0.15)As buffer layer 6 (with a
thickness of 5 .mu.m and a carrier concentration of
2.times.10.sup.18/cm.sup.3), and the p-type (Zn-doped)
Al.sub.0.1Ga.sub.0.9As contact layer 7 (with a thickness of 3 nm
and a carrier concentration of 7.5.times.10.sup.19/cm.sup.3) are
sequentially grown in lamination.
[0253] The growth temperature in the MOVPE growth is set at
650.degree. C. from the n-type GaAs buffer layer 2 to the p-type
buffer layer 6, and the growth temperature of the p-type contact
layer 7 is set at 550.degree. C. The other growth conditions are a
growth pressure of about 6666 Pa (50 Torr), a growth rate of the
respective layers in the range of 0.3 to 1.1 nm/sec, and a V/III
ratio of about 150. However, the p-type contact layer 7 has a V/III
ratio of 10. The p-type buffer layer 6 is made of AlGaAs with an Al
composition of about 0.8 to 0.9 and the V/III ratio during the
growth (in this case, about 50 in V/III ratio) is set such that the
C (carbon) concentration of the p-type buffer layer 6 is about
1.times.10.sup.18/cm.sup.3. Herein, the V/III ratio is a ratio
(quotient) obtained by a denominator, which is the number of moles
of a group III material such as TMGa or TMAl, and a numerator,
which is the number of moles of a group V material such as
AsH.sub.3 or PH.sub.3.
[0254] A material used in the MOVPE growth can be an organic metal
such as trimethyl gallium (TMGa) or triethyl gallium (TEGa) for Ga
source, trimethyl aluminum (TMAl) for Al source and trimethyl
indium (TMIn) for In source, or a hydride gas such as arsine
(AsH.sub.3) for As source and phosphine (PH.sub.3) for P source. A
dopant material for an n-type layer such as the n-type buffer layer
2 can be disilane (Si.sub.2H.sub.6). A dopant material for a
conductivity-determining impurity of a p-type layer such as the
p-type clad layer 5 and the p-type buffer layer 6 can be
biscyclopentadienyl magnesium (Cp.sub.2Mg). However, diethyl zinc
(DEZn) is used only for the p-type contact layer 7.
[0255] Furthermore, a dopant material for a
conductivity-determining impurity of an n-type layer can be also
hydrogen selenide (H.sub.2Se), monosilane (SiH.sub.4), diethyl
tellurium (DETe) and dimethyl tellurium (DMTe) A Zn source can be
also dimethyl zinc (DMZn).
[0256] Then, after the LED epitaxial wafer is taken out from the
MOVPE furnace, the ITO film 8 with a thickness of about 290 nm is
formed by the vacuum deposition method on the surface of the wafer,
i.e., the upper surface side of the p-type contact layer 7. In this
structure, the ITO film 8 composes the current spreading layer.
[0257] At this time, an evaluating glass substrate set in the same
batch as for the deposition of the ITO film 8 is taken out and cut
into a size enough for the Hall measurement, and the electric
characteristics of only the ITO film 8 are evaluated. As a result,
a carrier concentration is 1.05.times.10.sup.21/cm.sup.3, a
mobility is 20.3 cm.sup.2/Vs, and a resistivity is
2.94.times.10.sup.-4.OMEGA.cm.
[0258] Then, the surface electrode 9 as a circular electrode and
with a diameter of about 110 .mu.m is provided in the form of a dot
matrix on the upper surface of the epitaxial wafer by the vacuum
deposition method by using tools or materials and process used for
a general photolithography process such as a resist and a mask
aligner. A liftoff method is used in electrode formation after the
deposition. The surface electrode 9 is formed by sequentially
depositing Ni (nickel) and Au (gold) with thicknesses of 20 nm and
500 nm, respectively. Furthermore, on the entire bottom surface of
the epitaxial wafer, the back surface electrode 10 is formed by the
same vacuum deposition method. The back surface electrode 10 is
formed by sequentially depositing AuGe (gold-germanium alloy,
germanium content of 7.4%), Ni (nickel), and Au (gold) with
thicknesses of 60 nm, 10 nm, and 500 nm, respectively. Then, an
alloy process to alloy the electrode is performed such that the
electrode is heated at 440.degree. C. in a nitrogen gas atmosphere
for 5 minutes.
[0259] Then, the LED epitaxial wafer with the electrode formed as
described above is cut by a dicer such that the circular surface
electrode 9 is located at the center, so as to obtain an LED bear
chip with a chip size of 300 .mu.m square. Then, the LED bear chip
is mounted (die-bonding) through an Ag paste on a TO-18 stem. Then,
the LED bear chip is wire-bonded to have the LED device.
[0260] Initial characteristics of the LED device thus fabricated
are evaluated. As a result, the LED device has excellent initial
characteristics, i.e., an optical output of 0.99 mW and an
operating voltage of 1.842 V during the power feeding at 20 mA (in
evaluation).
[0261] Furthermore, a continuous current test for 168 hours (=1
week) is conducted where the LED device is driven at 50 mA in the
environment of room temperature (about 23.degree. C.) and
atmospheric humidity (about 40%). As relative values as compared to
those before the test, optical output is 102.1% (provided that
optical output before the power feeding is 100%; hereinafter
referred to as a relative output), and operation voltage is 1.843 V
(about 0.1% increased).
[0262] SIMS analysis is conducted for the LED device just after the
formation of the LED device and for the LED device after the
continuous current test under the above conditions.
[0263] FIG. 12 is a diagram showing the result of SIMS analysis
after the continuous current test. Meanwhile, the LED device sample
as used in FIG. 12 is prepared such that its surface is removed
several micrometers by mechanical polishing so as to enhance the
measurement resolution of the SIMS analysis.
[0264] As the result of the SIMS analysis, it is confirmed that the
C (carbon) concentration of the AlGaAs buffer layer 6 is about
1.0.times.10.sup.18/cm.sup.3 both before and after the continuous
current test, and that, in the LED device of Example 5 after the
continuous current test, Zn is not diffused into the active layer
4.
[0265] In consideration of the above results, the C concentration
of the buffer layer 6 is measured by the SIMS analysis while
varying the V/III ratio during the formation of the AlGaAs buffer
layer 6. FIG. 13B shows the measurement result. As shown in FIG.
13B, it is confirmed that the C concentration of the buffer layer 6
well correlates with the V/III ratio during the formation of the
AlGaAs buffer layer 6. However, the C concentration of the buffer
layer 6 is not determined simply by the V/III ratio, and it also
depends on the growth temperature or the Al composition of the
buffer layer 6. Thus, the control of the C concentration is not
limited only to the V/III ratio.
[0266] Further, using sample's fabricated varying the V/III ratio
as shown in FIG. 13B, the diffusion length of Zn doped in the
contact layer 7 in the LED structure as described in Example 5 is
measured. The relationship between the C concentration of the
AlGaAs buffer layer and the Zn diffusion length is shown in FIG.
2B. FIG. 2B shows that the Zn diffusion length depends on the C
concentration of the AlGaAs buffer layer. This result is
represented by the expression:
L=6.872.times.10.sup.-14.times.N.sub.C.sup.0.733, where N.sub.C is
a C (carbon) concentration [cm.sup.-3] of the buffer layer 6 and L
is a diffusion length [.mu.m] of a dopant doped in the p-type
contact layer 7. When the AlGaAs buffer layer 6 has a thickness of
more than the diffusion length L obtained by the expression, Zn
doped in the contact layer 7 is not diffused into the active layer
4 so that the LED device can be excellent both in initial property
and reliability. Thus, the excellent properties of the LED device
in Example 5 are based on the above reasons. Further, almost all of
the LED devices in Example 5 can be fabricated without being
broken.
Example 6
[0267] In Example 6, a red LED epitaxial wafer with a structure as
shown in FIG. 3 and an emission wavelength of about 630 nm is
fabricated. Its epitaxial growth method, epitaxial layer thickness,
epitaxial layer structure, electrode formation method, and LED
device fabrication method are basically the same as those in
Example 5 (FIG. 1). Different points from Example 5 will be
described below.
[0268] Example 6 has the feature that a device structure is
employed that a semiconductor layer not positively doped, so-called
undoped layer, is formed as the diffusion suppressing layer 11
between the active layer 4 and the p-type cladding layer 5. The
diffusion suppressing layer 11 serves to prevent the p-type dopant
doped into the upper p-type semiconductor layer including the
p-type cladding layer 5 from penetrating into the active layer 4.
The layer 11 has the same composition as the p-type cladding layer
5, and a thickness of 100 nm.
[0269] In addition, as a reference example for Example 6, an LED
epitaxial wafer is fabricated such that the diffusion suppressing
layer 11 is similarly inserted into an LED device as described
later in Comparative Example 3 (FIG. 9).
[0270] Then, the LED epitaxial wafers thus fabricated are formed
into devices as in Example 5.
[0271] Initial characteristics of the LED device thus fabricated
are evaluated. As a result, the LED device in the reference example
has excellent initial characteristics, i.e., an optical output of
0.96 mW and an operating voltage of 1.854 V during the power
feeding at 20 mA (in evaluation).
[0272] Further, the continuous current test is conducted on the
same conditions as Example 5. As a result, the LED device in the
reference example has a relative output of 76.1% and an operating
voltage of 1.904 V (about 2.6% increased), where the relative
output is enhanced as compared to Comparative Example 3 described
later.
[0273] Then, initial characteristics of the LED device in Example
6, i.e., the LED device with the diffusion suppressing layer 11
added to the LED structure in Example 5 as shown in FIG. 3, is
evaluated. As a result, the LED device in Example 6 has excellent
initial characteristics, i.e., an optical output of 0.97 mW and an
operating voltage of 1.843 V during the power feeding at 20 mA (in
evaluation).
[0274] Further, for the LED device in Example 6, the continuous
current test is conducted on the same conditions as Example 5. As a
result, the LED device has a relative output of 101.3% and an
operating voltage of 1.843 V (no change).
[0275] As described above, although the diffusion suppressing layer
11 used in Example 6 does not completely suppress the diffusion of
Zn, the diffusion suppressing layer 11 can serve to reduce the
amount of Zn being diffused into the active layer 4. As a result,
the LED device in the reference example can have the relative
output enhanced as compared to Comparative Example 3 described
later. Thus, even when the invention is applied to a device
structure not subjected to the diffusion of Zn, the diffusion
suppressing layer 11 can achieve the similar effect without causing
any adverse effect.
Example 7
[0276] In Example 7, a red LED epitaxial wafer with a structure as
shown in FIG. 4 and an emission wavelength of about 630 nm is
fabricated. Its epitaxial growth method, epitaxial layer thickness,
epitaxial layer structure, electrode formation method, and LED
device fabrication method are basically the same as those in
Example 5 (FIG. 1).
[0277] Example 7 has the feature that a light reflecting layer 12
is formed between the n-type GaAs buffer layer 2 and the n-type
cladding layer 3 such that it comprises 20 pairs of DBR
(distributed Bragg reflector) where an n-type AlInP layers and an
n-type Al.sub.0.4Ga.sub.0.6As layer, 20 layers respectively, are
alternately formed.
[0278] The light reflecting layer 12 has a thickness obtained by
.lamda..sub.p/(4.times.n )where .lamda..sub.p is an emission peak
wavelength (nm) of the LED device and n is a refractive index of
the semiconductor material to compose the light reflecting layer
12. The light reflecting layer 12 has uniformly a carrier
concentration of about 1.times.10.sup.18/cm.sup.3.
[0279] Initial characteristics of the LED device thus fabricated
are evaluated. As a result, the LED device in Example 7 has
excellent initial characteristics, i.e., an optical output of 1.53
mW and an operating voltage of 1.855 V during the power feeding at
20 mA (in evaluation). Further, as the result of the continuous
current test, the LED device in Example 7 has a relative output of
101.6% and an operating voltage of 1.856 V (about 0.1%
increased).
[0280] As described above, in addition to the effects of Example 5,
the LED device in Example 7 can have an optical output higher than
that of Example 5 by providing the light reflecting layer 12 of
semiconductor multilayer between the n-type buffer layer 2 and the
n-type cladding layer 3. This effect is produced by the effective
light extraction efficiency enhanced by the light reflecting layer
12.
Example 8
[0281] In Example 8, a red LED epitaxial wafer with a structure as
shown in FIG. 5 and an emission wavelength of about 630 nm is
fabricated. Its epitaxial growth method, epitaxial layer thickness,
epitaxial layer structure, electrode formation method, and LED
device fabrication method are basically the same as those in
Example 5 (FIG. 1).
[0282] Example 8 has the feature that a MQW (multiquantum well)
active layer 13 is used instead of the active layer 4. The MQW is
composed of 40.5 pairs of a
(Al.sub.0.5Ga.sub.0.5).sub.0.5In.sub.0.5P (about 7.5 nm thick)
barrier layer and a Ga.sub.0.5In.sub.0.5P (about 5.5 nm thick) well
layer.
[0283] A modification of Example 8 is an LED (not shown) that the
composition ratio of Ga and In in the well layer of the MQW active
layer 13 is changed to have a strained multiquantum well structure
so that a compressive or tensile stress is applied to a start GaAs
substrate. The well layer with the strained multiquantum well
structure in the modification is composed such that the Ga
composition is reduced and the In composition is increased by that
difference, and it is subjected to a compressive strain caused by a
lattice mismatch that the lattice constant of the well layer is
different from an underlying layer such as the n-type cladding
layer 3.
[0284] The two LED devices (Example 8 and the modification) thus
fabricated respectively have excellent initial characteristics,
i.e., optical outputs of 1.14 and 1.23 mW and operating voltages of
1.844 and 1.843 V during power feeding at 20 mA (in
evaluation).
[0285] Further, when the continuous current test is conducted under
the same conditions as Example 5, the two LED devices have relative
outputs of 102.5% and 102.3%, respectively.
[0286] As described above, in Example 8 and the modification each
employing the MQW and strained multiquantum well structure instead
of the active layer 4 of Example 5, each optical output is
increased as compared to Example 5. Thus, by employing the quantum
well structure, the internal quantum efficiency of the LED device
can be increased to enhance the total characteristics of the LED
device. Further, the features of Example 5 can be applied to the
quantum well structure of Example 8 and the modification.
[0287] In Examples 5-8 of the invention, the red LED device with an
emission wavelength of 630 nm is fabricated. However, other LED
devices made of the same AlGaInP-based material with an emission
wavelength of 560 to 660 nm can also have the effects of the
invention by setting suitably the C (carbon) concentration and the
thickness of the buffer layer.
[0288] In Examples 5-8 of the invention, the LED device structure
is employed that the buffer layer 2 is formed between the GaAs
substrate land the n-type cladding layer 3. However, even when the
n-type cladding layer 3 is formed directly on the GaAs substrate 1,
the effects of the invention can be obtained.
[0289] Although Examples 5-8 have the circular surface electrode 9,
the other electrode shape such as rectangular, rhombic and
polygonal, and further the electrode shape being accompanied with a
wing-shaped or branch-shaped portion can be used. Electrodes with
such a shape can also have the effects of the invention.
[0290] In Examples 5-8, the semiconductor substrate comprises GaAs.
Alternatively, an LED epitaxial wafer may comprise a Ge substrate,
or GaAs or Ge substrate as a start substrate and then replaced by
Si or a metallic self-standing substrate with a higher thermal
conductivity than Si. Thereby, the effects of the invention can be
obtained.
[0291] In Examples 5-8, the buffer layer 6 comprises AlGaAs which
has an Al composition in the range of about 0.8 to 0.9. However,
the invention is not limited to the Al composition. Even when the
Al composition of the AlGaAs buffer layer 6 is not in the above
range, or even when the V/III ratio is not in the range of Examples
5-8, there is no problem if only the C (carbon) concentration and
the thickness of the buffer layer 6 are suitably set according to
the invention.
[0292] For example, in case of an LED device with an emission
wavelength of about 650 nm, even when the Al composition of the
AlGaAs buffer layer 6 is in the range of about 0.6 to 0.7, light
emitted from the active layer is little absorbed so that the LED
device can have a high optical output. In addition, since the C
concentration of the buffer layer 6 decreases directly in
connection with the reduction in Al composition, the V/III ratio
can be set lower than Examples 5-8 using the buffer layer 6 with an
Al composition of 0.8 to 0.9.
[0293] In case of an LED device with an emission wavelength of
about 570 nm, in order to eliminate the optical absorption to be
caused by the buffer layer 6, the Al composition of the buffer
layer 6 can be set to be around 0.9 so that the V/III ratio can be
higher than the conventional one. Thus, even when the fabrication
parameters such as a composition of material composing the buffer
layer, a V/III ratio during the epitaxial growth or a growth
temperature are changed, the effects of the invention can be
obtained by satisfying the requirements of the invention.
[0294] In Examples 5-8, the current-spreading layer 8 comprises
ITO. Instead of the ITO, a typical transparent conductive film such
as ZnO and CTO film with a high transmittance for visible light and
a low electrical resistivity can be used as the current-spreading
layer 8. However, a more important factor for a material used as
the current-spreading layer than the above two factors is
enhancement in carrier concentration. Since the importance of the
carrier concentration is as described above, materials applicable
to the current-spreading layer will be limited to some extent in
consideration of a reduction in the operating voltage of the LED
device. Thus, a suitable material should be selected from the
materials.
Comparative Example 3
[0295] In Comparative Example 3, a red LED epitaxial wafer with a
structure as shown in FIG. 9 and an emission wavelength of about
630 nm is fabricated. Its epitaxial growth method, epitaxial layer
thickness, epitaxial layer structure, electrode formation method,
and LED device fabrication method are basically the same as those
in Example 5 (FIG. 1). Different points from Example 5 will be
described below.
[0296] Comparative Example 3 is prepared such that the V/III ratio
during the growth of a p-type buffer layer 16 is 10 and the C
concentration of the p-type buffer layer 16 is
1.8.times.10.sup.19/cm.sup.3.
[0297] Then, the LED epitaxial wafer thus fabricated is formed into
devices as in Example 5.
[0298] Initial characteristics of the LED device thus fabricated
are evaluated. As a result, the LED device in Comparative Example 3
has initial characteristics, i.e., an optical output of 0.92 mW and
an operating voltage of 1.855 V during the power feeding at 20 mA
(in evaluation).
[0299] Further, the continuous current test is conducted on the
same conditions as Example 5. As a result, the LED device in
Comparative Example 3 has a relative output of 52% and an operating
voltage of 1.915 V (about 3% increased).
[0300] SIMS analysis is conducted for the LED device just after the
formation of the LED device and for the LED device after the
continuous current test under the above conditions.
[0301] FIG. 11 is a diagram showing the result of SIMS analysis
after the continuous current test. Meanwhile, the LED device sample
as used in FIG. 11 is prepared such that its surface is removed
several micrometers by mechanical polishing so as to enhance the
measurement resolution of the SIMS analysis.
[0302] As the result of the SIMS analysis, it is confirmed that the
C (carbon) concentration of the buffer layer 16 is
1.8.times.10.sup.19/cm.sup.3 both before and after the continuous
current test, and that, in the LED device of Comparative Example 3
after the continuous current test, Zn doped as a dopant into the
p-type contact layer 7 is diffused into the active layer 4. Thus,
deterioration in lifetime, i.e., reliability in the LED device of
Comparative Example 3 is caused by the dopant diffusion.
Comparative Example 4
[0303] In Comparative Example 4, a red LED epitaxial wafer with a
structure as shown in FIG. 10 and an emission wavelength of about
630 nm is fabricated. Its epitaxial growth method, epitaxial layer
thickness, epitaxial layer structure, electrode formation method,
and LED device fabrication method are basically the same as those
in Comparative Example 3 (FIG. 9) Different points from Comparative
Example 3 will be described below.
[0304] Comparative Example 4 is not formed with the p-type buffer
layer 16. The p-type cladding layer 5 has a thickness of about 400
nm, which is enough to have a carrier confining effect and to serve
as a carrier (hole) supply layer. Namely, the p-type cladding layer
5 with a thickness of about 400 nm can sufficiently serve as a
cladding layer. Thus, the LED device of Comparative example 4 has
the same structure as Comparative Example 3 except not having the
AlGaAs buffer layer 16.
[0305] Then, the LED epitaxial wafer thus fabricated is formed into
devices as in Comparative Example 3.
[0306] Initial characteristics of the LED device thus fabricated
are evaluated. As a result, the LED device in Comparative Example 4
has initial characteristics, i.e., an optical output of 0.89 mW and
an operating voltage of 1.840 V during the power feeding at 20 mA
(in evaluation).
[0307] However, in evaluating the initial characteristics, about
20-30% of the devices are broken so that it does not emit light.
Although the device not broken has the abovementioned
characteristics, the about 20-30% of the devices broken cannot be
evaluated. This is assumed because the device is broken in the wire
bonding process before the device evaluation. When the continuous
current test is conducted for the devices not broken under the same
conditions as Comparative Example 3, the LED device has a relative
output of 79% and an operating voltage of 1.850 V (about 0.5%
increased).
[0308] As described above, in case of the structure without the
buffer layer, the product yield deteriorates and the optical output
and reliability is not sufficient. Namely, although the relative
output is a little improved as compared to Comparative Example 3,
the product yield is contrary reduced (In Comparative Example 3,
none of the LED devices is broken).
[0309] Although the invention has been described with respect to
the specific embodiments for complete and clear disclosure, the
appended claims are not to be thus limited but are to be construed
as embodying all modifications and alternative constructions that
may occur to one skilled in the art which fairly fall within the
basic teaching herein set forth.
* * * * *